add comment about fts_open

Sources/FoundationEssentials/FileManager/FileOperations+Enumeration.swift

@@ -168,6 +168,10 @@ struct _FTSSequence: Sequence {
 
             state = [Optional(UnsafeMutablePointer(mutating: path)), nil].withUnsafeBufferPointer { dirList in
 #if canImport(Android)
+                // The first parameter in `fts_open` in the Android NDK takes in a
+                // non-null char pointer, even though the last element in the array of pointers
+                // should be null. This cast resolves this nullability annotation that appears
+                // to be incorrect in the NDK.
                 let baseAddress = unsafeBitCast(dirList.baseAddress!, to: UnsafePointer<UnsafeMutablePointer<CChar>>.self)
 #else
                 let baseAddress = dirList.baseAddress!