Fix comment

velox/docs/develop/scalar-functions.rst

@@ -581,14 +581,8 @@ process. This set may not include all the rows. By default, a vector function
 is assumed to have the default null behavior, e.g. null in any input produces
 a null result. In this case, the expression evaluation engine will exclude
 rows with nulls from the “rows” specified in the call to “apply”. If a function
-has a different behavior for null inputs, metadata flag must be provided with
-defaultNullBehavior set to false during registration. See the below "Registration"
-section.
-
-.. code-block:: c++
-
-    VectorFunctionMetadata metadata =
-      exec::VectorFunctionMetadataBuilder().defaultNullBehavior(false).build();
+has a different behavior for null inputs, it must specify that during registration.
+See :ref:`vector function registration<Registration>` for more details.
 
 In this case, the “rows” parameter will include rows with null inputs and the
 function will need to handle these. By default, the function can assume that
@@ -641,14 +635,9 @@ of the function arguments. These vectors are not necessarily flat and may be
 dictionary or constant encoded. However, a deterministic function that takes
 a single argument and has default null behavior is guaranteed to receive its
 only input as a flat or constant vector. By default, a function is assumed to
-be deterministic. If that’s not the case, metadata flag must be provided with
-deterministic set to false during registration. See the below "Registration"
-section.
-
-.. code-block:: c++
-
-    VectorFunctionMetadata metadata =
-      exec::VectorFunctionMetadataBuilder().deterministic(false).build();
+be deterministic. If that’s not the case, it must specify the non-deterministic
+behavior during registration. See :ref:`vector function registration<Registration>`
+for more details.
 
 Note that :ref:`decoded-vector` can be used to get a flat vector-like interface to any
 vector. A helper class exec::DecodedArgs can be used to decode multiple arguments.
@@ -859,6 +848,7 @@ Simple functions are compatible with the TRY expression by default. The framewor
 wraps the “call” and “callNullable” methods in a try-catch and reports errors
 using context.setError.
 
+.. _Registration:
 Registration
 ^^^^^^^^^^^^
 
@@ -874,9 +864,17 @@ Use exec::registerVectorFunction to register a stateless vector function.
         bool overwrite = true)
 
 exec::registerVectorFunction takes a name, a list of supported signatures
-and unique_ptr to an instance of the function. It is optional to provide
-metadata flag to set deterministic or defaultNullBehavior. An optional
-“overwrite” flag specifies whether to overwrite a function if a function
+and unique_ptr to an instance of the function. It takes an optional 'metadata'
+parameter that specifies whether a function is deterministic, has default null
+behavior, and other properties. A helper VectorFunctionMetadataBuilder class
+allows to easily construct 'metadata'. For example,
+
+.. code-block:: c++
+
+    VectorFunctionMetadataBuilder().defaultNullBehavior(false).build();
+
+
+An optional “overwrite” flag specifies whether to overwrite a function if a function
 with the specified name already exists.
 
 Use exec::registerStatefulVectorFunction to register a stateful vector