[SPARK-18615][DOCS] Switch to multi-line doc to avoid a genjavadoc bu…

…g for backticks

## What changes were proposed in this pull request?

Currently, single line comment does not mark down backticks to `<code>..</code>` but prints as they are (`` `..` ``). For example, the line below:

```scala
/** Return an RDD with the pairs from `this` whose keys are not in `other`. */
```

So, we could work around this as below:

```scala
/**
 * Return an RDD with the pairs from `this` whose keys are not in `other`.
 */
```

- javadoc

  - **Before**
    ![2016-11-29 10 39 14](https://cloud.githubusercontent.com/assets/6477701/20693606/e64c8f90-b622-11e6-8dfc-4a029216e23d.png)

  - **After**
    ![2016-11-29 10 39 08](https://cloud.githubusercontent.com/assets/6477701/20693607/e7280d36-b622-11e6-8502-d2e21cd5556b.png)

- scaladoc (this one looks fine either way)

  - **Before**
    ![2016-11-29 10 38 22](https://cloud.githubusercontent.com/assets/6477701/20693640/12c18aa8-b623-11e6-901a-693e2f6f8066.png)

  - **After**
    ![2016-11-29 10 40 05](https://cloud.githubusercontent.com/assets/6477701/20693642/14eb043a-b623-11e6-82ac-7cd0000106d1.png)

I suspect this is related with SPARK-16153 and genjavadoc issue in ` lightbend/genjavadoc#85`.

## How was this patch tested?

I found them via

```
grep -r "\/\*\*.*\`" . | grep .scala
````

and then checked if each is in the public API documentation with manually built docs (`jekyll build`) with Java 7.

Author: hyukjinkwon <[email protected]>

Closes #16050 from HyukjinKwon/javadoc-markdown.

core/src/main/scala/org/apache/spark/SparkConf.scala

@@ -378,7 +378,9 @@ class SparkConf(loadDefaults: Boolean) extends Cloneable with Logging with Seria
     settings.entrySet().asScala.map(x => (x.getKey, x.getValue)).toArray
   }
 
-  /** Get all parameters that start with `prefix` */
+  /**
+   * Get all parameters that start with `prefix`
+   */
   def getAllWithPrefix(prefix: String): Array[(String, String)] = {
     getAll.filter { case (k, v) => k.startsWith(prefix) }
       .map { case (k, v) => (k.substring(prefix.length), v) }

core/src/main/scala/org/apache/spark/api/java/JavaDoubleRDD.scala

@@ -45,7 +45,9 @@ class JavaDoubleRDD(val srdd: RDD[scala.Double])
 
   import JavaDoubleRDD.fromRDD
 
-  /** Persist this RDD with the default storage level (`MEMORY_ONLY`). */
+  /**
+   * Persist this RDD with the default storage level (`MEMORY_ONLY`).
+   */
   def cache(): JavaDoubleRDD = fromRDD(srdd.cache())
 
   /**

core/src/main/scala/org/apache/spark/api/java/JavaPairRDD.scala

@@ -54,7 +54,9 @@ class JavaPairRDD[K, V](val rdd: RDD[(K, V)])
 
   // Common RDD functions
 
-  /** Persist this RDD with the default storage level (`MEMORY_ONLY`). */
+  /**
+   * Persist this RDD with the default storage level (`MEMORY_ONLY`).
+   */
   def cache(): JavaPairRDD[K, V] = new JavaPairRDD[K, V](rdd.cache())
 
   /**
@@ -454,13 +456,17 @@ class JavaPairRDD[K, V](val rdd: RDD[(K, V)])
     fromRDD(rdd.subtractByKey(other))
   }
 
-  /** Return an RDD with the pairs from `this` whose keys are not in `other`. */
+  /**
+   * Return an RDD with the pairs from `this` whose keys are not in `other`.
+   */
   def subtractByKey[W](other: JavaPairRDD[K, W], numPartitions: Int): JavaPairRDD[K, V] = {
     implicit val ctag: ClassTag[W] = fakeClassTag
     fromRDD(rdd.subtractByKey(other, numPartitions))
   }
 
-  /** Return an RDD with the pairs from `this` whose keys are not in `other`. */
+  /**
+   * Return an RDD with the pairs from `this` whose keys are not in `other`.
+   */
   def subtractByKey[W](other: JavaPairRDD[K, W], p: Partitioner): JavaPairRDD[K, V] = {
     implicit val ctag: ClassTag[W] = fakeClassTag
     fromRDD(rdd.subtractByKey(other, p))

core/src/main/scala/org/apache/spark/api/java/JavaRDD.scala

@@ -34,7 +34,9 @@ class JavaRDD[T](val rdd: RDD[T])(implicit val classTag: ClassTag[T])
 
   // Common RDD functions
 
-  /** Persist this RDD with the default storage level (`MEMORY_ONLY`). */
+  /**
+   * Persist this RDD with the default storage level (`MEMORY_ONLY`).
+   */
   def cache(): JavaRDD[T] = wrapRDD(rdd.cache())
 
   /**

core/src/main/scala/org/apache/spark/rdd/PairRDDFunctions.scala

@@ -914,14 +914,18 @@ class PairRDDFunctions[K, V](self: RDD[(K, V)])
     subtractByKey(other, self.partitioner.getOrElse(new HashPartitioner(self.partitions.length)))
   }
 
-  /** Return an RDD with the pairs from `this` whose keys are not in `other`. */
+  /**
+   * Return an RDD with the pairs from `this` whose keys are not in `other`.
+   */
   def subtractByKey[W: ClassTag](
       other: RDD[(K, W)],
       numPartitions: Int): RDD[(K, V)] = self.withScope {
     subtractByKey(other, new HashPartitioner(numPartitions))
   }
 
-  /** Return an RDD with the pairs from `this` whose keys are not in `other`. */
+  /**
+   * Return an RDD with the pairs from `this` whose keys are not in `other`.
+   */
   def subtractByKey[W: ClassTag](other: RDD[(K, W)], p: Partitioner): RDD[(K, V)] = self.withScope {
     new SubtractedRDD[K, V, W](self, other, p)
   }

core/src/main/scala/org/apache/spark/rdd/RDD.scala

@@ -195,10 +195,14 @@ abstract class RDD[T: ClassTag](
     }
   }
 
-  /** Persist this RDD with the default storage level (`MEMORY_ONLY`). */
+  /**
+   * Persist this RDD with the default storage level (`MEMORY_ONLY`).
+   */
   def persist(): this.type = persist(StorageLevel.MEMORY_ONLY)
 
-  /** Persist this RDD with the default storage level (`MEMORY_ONLY`). */
+  /**
+   * Persist this RDD with the default storage level (`MEMORY_ONLY`).
+   */
   def cache(): this.type = persist()
 
   /**

graphx/src/main/scala/org/apache/spark/graphx/impl/EdgeRDDImpl.scala

@@ -63,7 +63,9 @@ class EdgeRDDImpl[ED: ClassTag, VD: ClassTag] private[graphx] (
     this
   }
 
-  /** Persists the edge partitions using `targetStorageLevel`, which defaults to MEMORY_ONLY. */
+  /**
+   * Persists the edge partitions using `targetStorageLevel`, which defaults to MEMORY_ONLY.
+   */
   override def cache(): this.type = {
     partitionsRDD.persist(targetStorageLevel)
     this

graphx/src/main/scala/org/apache/spark/graphx/impl/GraphImpl.scala

@@ -277,7 +277,9 @@ class GraphImpl[VD: ClassTag, ED: ClassTag] protected (
 
 object GraphImpl {
 
-  /** Create a graph from edges, setting referenced vertices to `defaultVertexAttr`. */
+  /**
+   * Create a graph from edges, setting referenced vertices to `defaultVertexAttr`.
+   */
   def apply[VD: ClassTag, ED: ClassTag](
       edges: RDD[Edge[ED]],
       defaultVertexAttr: VD,
@@ -286,7 +288,9 @@ object GraphImpl {
     fromEdgeRDD(EdgeRDD.fromEdges(edges), defaultVertexAttr, edgeStorageLevel, vertexStorageLevel)
   }
 
-  /** Create a graph from EdgePartitions, setting referenced vertices to `defaultVertexAttr`. */
+  /**
+   * Create a graph from EdgePartitions, setting referenced vertices to `defaultVertexAttr`.
+   */
   def fromEdgePartitions[VD: ClassTag, ED: ClassTag](
       edgePartitions: RDD[(PartitionID, EdgePartition[ED, VD])],
       defaultVertexAttr: VD,
@@ -296,7 +300,9 @@ object GraphImpl {
       vertexStorageLevel)
   }
 
-  /** Create a graph from vertices and edges, setting missing vertices to `defaultVertexAttr`. */
+  /**
+   * Create a graph from vertices and edges, setting missing vertices to `defaultVertexAttr`.
+   */
   def apply[VD: ClassTag, ED: ClassTag](
       vertices: RDD[(VertexId, VD)],
       edges: RDD[Edge[ED]],

graphx/src/main/scala/org/apache/spark/graphx/impl/VertexRDDImpl.scala

@@ -63,7 +63,9 @@ class VertexRDDImpl[VD] private[graphx] (
     this
   }
 
-  /** Persists the vertex partitions at `targetStorageLevel`, which defaults to MEMORY_ONLY. */
+  /**
+   * Persists the vertex partitions at `targetStorageLevel`, which defaults to MEMORY_ONLY.
+   */
   override def cache(): this.type = {
     partitionsRDD.persist(targetStorageLevel)
     this

mllib-local/src/main/scala/org/apache/spark/ml/linalg/Matrices.scala

@@ -85,25 +85,33 @@ sealed trait Matrix extends Serializable {
   @Since("2.0.0")
   def copy: Matrix
 
-  /** Transpose the Matrix. Returns a new `Matrix` instance sharing the same underlying data. */
+  /**
+   * Transpose the Matrix. Returns a new `Matrix` instance sharing the same underlying data.
+   */
   @Since("2.0.0")
   def transpose: Matrix
 
-  /** Convenience method for `Matrix`-`DenseMatrix` multiplication. */
+  /**
+   * Convenience method for `Matrix`-`DenseMatrix` multiplication.
+   */
   @Since("2.0.0")
   def multiply(y: DenseMatrix): DenseMatrix = {
     val C: DenseMatrix = DenseMatrix.zeros(numRows, y.numCols)
     BLAS.gemm(1.0, this, y, 0.0, C)
     C
   }
 
-  /** Convenience method for `Matrix`-`DenseVector` multiplication. For binary compatibility. */
+  /**
+   * Convenience method for `Matrix`-`DenseVector` multiplication. For binary compatibility.
+   */
   @Since("2.0.0")
   def multiply(y: DenseVector): DenseVector = {
     multiply(y.asInstanceOf[Vector])
   }
 
-  /** Convenience method for `Matrix`-`Vector` multiplication. */
+  /**
+   * Convenience method for `Matrix`-`Vector` multiplication.
+   */
   @Since("2.0.0")
   def multiply(y: Vector): DenseVector = {
     val output = new DenseVector(new Array[Double](numRows))

mllib/src/main/scala/org/apache/spark/ml/Pipeline.scala

@@ -216,7 +216,9 @@ object Pipeline extends MLReadable[Pipeline] {
     }
   }
 
-  /** Methods for `MLReader` and `MLWriter` shared between [[Pipeline]] and [[PipelineModel]] */
+  /**
+   * Methods for `MLReader` and `MLWriter` shared between [[Pipeline]] and [[PipelineModel]]
+   */
   private[ml] object SharedReadWrite {
 
     import org.json4s.JsonDSL._

mllib/src/main/scala/org/apache/spark/ml/attribute/AttributeGroup.scala

@@ -239,7 +239,9 @@ object AttributeGroup {
     }
   }
 
-  /** Creates an attribute group from a `StructField` instance. */
+  /**
+   * Creates an attribute group from a `StructField` instance.
+   */
   def fromStructField(field: StructField): AttributeGroup = {
     require(field.dataType == new VectorUDT)
     if (field.metadata.contains(ML_ATTR)) {

mllib/src/main/scala/org/apache/spark/ml/attribute/attributes.scala

@@ -109,7 +109,9 @@ sealed abstract class Attribute extends Serializable {
     StructField(name.get, DoubleType, nullable = false, newMetadata)
   }
 
-  /** Converts to a `StructField`. */
+  /**
+   * Converts to a `StructField`.
+   */
   def toStructField(): StructField = toStructField(Metadata.empty)
 
   override def toString: String = toMetadataImpl(withType = true).toString
@@ -369,12 +371,16 @@ class NominalAttribute private[ml] (
   override def withIndex(index: Int): NominalAttribute = copy(index = Some(index))
   override def withoutIndex: NominalAttribute = copy(index = None)
 
-  /** Copy with new values and empty `numValues`. */
+  /**
+   * Copy with new values and empty `numValues`.
+   */
   def withValues(values: Array[String]): NominalAttribute = {
     copy(numValues = None, values = Some(values))
   }
 
-  /** Copy with new values and empty `numValues`. */
+  /**
+   * Copy with new values and empty `numValues`.
+   */
   @varargs
   def withValues(first: String, others: String*): NominalAttribute = {
     copy(numValues = None, values = Some((first +: others).toArray))
@@ -385,12 +391,16 @@ class NominalAttribute private[ml] (
     copy(values = None)
   }
 
-  /** Copy with a new `numValues` and empty `values`. */
+  /**
+   * Copy with a new `numValues` and empty `values`.
+   */
   def withNumValues(numValues: Int): NominalAttribute = {
     copy(numValues = Some(numValues), values = None)
   }
 
-  /** Copy without the `numValues`. */
+  /**
+   * Copy without the `numValues`.
+   */
   def withoutNumValues: NominalAttribute = copy(numValues = None)
 
   /**

mllib/src/main/scala/org/apache/spark/ml/classification/LogisticRegression.scala

@@ -1105,7 +1105,9 @@ sealed trait LogisticRegressionTrainingSummary extends LogisticRegressionSummary
  */
 sealed trait LogisticRegressionSummary extends Serializable {
 
-  /** Dataframe output by the model's `transform` method. */
+  /**
+   * Dataframe output by the model's `transform` method.
+   */
   def predictions: DataFrame
 
   /** Field in "predictions" which gives the probability of each class as a vector. */

mllib/src/main/scala/org/apache/spark/ml/regression/GeneralizedLinearRegression.scala

@@ -886,7 +886,9 @@ class GeneralizedLinearRegressionSummary private[regression] (
   protected val model: GeneralizedLinearRegressionModel =
     origModel.copy(ParamMap.empty).setPredictionCol(predictionCol)
 
-  /** Predictions output by the model's `transform` method. */
+  /**
+   * Predictions output by the model's `transform` method.
+   */
   @Since("2.0.0") @transient val predictions: DataFrame = model.transform(dataset)
 
   private[regression] lazy val family: Family = Family.fromName(model.getFamily)

mllib/src/main/scala/org/apache/spark/mllib/feature/ChiSqSelector.scala

@@ -255,10 +255,14 @@ class ChiSqSelector @Since("2.1.0") () extends Serializable {
 
 private[spark] object ChiSqSelector {
 
-  /** String name for `numTopFeatures` selector type. */
+  /**
+   * String name for `numTopFeatures` selector type.
+   */
   val NumTopFeatures: String = "numTopFeatures"
 
-  /** String name for `percentile` selector type. */
+  /**
+   * String name for `percentile` selector type.
+   */
   val Percentile: String = "percentile"
 
   /** String name for `fpr` selector type. */

mllib/src/main/scala/org/apache/spark/mllib/linalg/Matrices.scala

@@ -91,25 +91,33 @@ sealed trait Matrix extends Serializable {
   @Since("1.2.0")
   def copy: Matrix
 
-  /** Transpose the Matrix. Returns a new `Matrix` instance sharing the same underlying data. */
+  /**
+   * Transpose the Matrix. Returns a new `Matrix` instance sharing the same underlying data.
+   */
   @Since("1.3.0")
   def transpose: Matrix
 
-  /** Convenience method for `Matrix`-`DenseMatrix` multiplication. */
+  /**
+   * Convenience method for `Matrix`-`DenseMatrix` multiplication.
+   */
   @Since("1.2.0")
   def multiply(y: DenseMatrix): DenseMatrix = {
     val C: DenseMatrix = DenseMatrix.zeros(numRows, y.numCols)
     BLAS.gemm(1.0, this, y, 0.0, C)
     C
   }
 
-  /** Convenience method for `Matrix`-`DenseVector` multiplication. For binary compatibility. */
+  /**
+   * Convenience method for `Matrix`-`DenseVector` multiplication. For binary compatibility.
+   */
   @Since("1.2.0")
   def multiply(y: DenseVector): DenseVector = {
     multiply(y.asInstanceOf[Vector])
   }
 
-  /** Convenience method for `Matrix`-`Vector` multiplication. */
+  /**
+   * Convenience method for `Matrix`-`Vector` multiplication.
+   */
   @Since("1.4.0")
   def multiply(y: Vector): DenseVector = {
     val output = new DenseVector(new Array[Double](numRows))

mllib/src/main/scala/org/apache/spark/mllib/linalg/distributed/BlockMatrix.scala

@@ -295,7 +295,9 @@ class BlockMatrix @Since("1.3.0") (
     new IndexedRowMatrix(rows)
   }
 
-  /** Collect the distributed matrix on the driver as a `DenseMatrix`. */
+  /**
+   * Collect the distributed matrix on the driver as a `DenseMatrix`.
+   */
   @Since("1.3.0")
   def toLocalMatrix(): Matrix = {
     require(numRows() < Int.MaxValue, "The number of rows of this matrix should be less than " +

mllib/src/main/scala/org/apache/spark/mllib/linalg/distributed/CoordinateMatrix.scala

@@ -101,7 +101,9 @@ class CoordinateMatrix @Since("1.0.0") (
     toIndexedRowMatrix().toRowMatrix()
   }
 
-  /** Converts to BlockMatrix. Creates blocks of `SparseMatrix` with size 1024 x 1024. */
+  /**
+   * Converts to BlockMatrix. Creates blocks of `SparseMatrix` with size 1024 x 1024.
+   */
   @Since("1.3.0")
   def toBlockMatrix(): BlockMatrix = {
     toBlockMatrix(1024, 1024)

mllib/src/main/scala/org/apache/spark/mllib/linalg/distributed/IndexedRowMatrix.scala

@@ -90,7 +90,9 @@ class IndexedRowMatrix @Since("1.0.0") (
     new RowMatrix(rows.map(_.vector), 0L, nCols)
   }
 
-  /** Converts to BlockMatrix. Creates blocks of `SparseMatrix` with size 1024 x 1024. */
+  /**
+   * Converts to BlockMatrix. Creates blocks of `SparseMatrix` with size 1024 x 1024.
+   */
   @Since("1.3.0")
   def toBlockMatrix(): BlockMatrix = {
     toBlockMatrix(1024, 1024)

mllib/src/main/scala/org/apache/spark/mllib/stat/Statistics.scala

@@ -176,7 +176,9 @@ object Statistics {
     ChiSqTest.chiSquaredFeatures(data)
   }
 
-  /** Java-friendly version of `chiSqTest()` */
+  /**
+   * Java-friendly version of `chiSqTest()`
+   */
   @Since("1.5.0")
   def chiSqTest(data: JavaRDD[LabeledPoint]): Array[ChiSqTestResult] = chiSqTest(data.rdd)
 
@@ -218,7 +220,9 @@ object Statistics {
     KolmogorovSmirnovTest.testOneSample(data, distName, params: _*)
   }
 
-  /** Java-friendly version of `kolmogorovSmirnovTest()` */
+  /**
+   * Java-friendly version of `kolmogorovSmirnovTest()`
+   */
   @Since("1.5.0")
   @varargs
   def kolmogorovSmirnovTest(