[SPARK-17264][SQL] DataStreamWriter should document that it only supports Parquet for now

## What changes were proposed in this pull request?

Clarify that only parquet files are supported by DataStreamWriter now

## How was this patch tested?

(Doc build -- no functional changes to test)

Author: Sean Owen <sowen@cloudera.com>

Closes #14860 from srowen/SPARK-17264.

python/pyspark/sql/streaming.py

@@ -591,7 +591,7 @@ class DataStreamWriter(object):
 
         .. note:: Experimental.
 
-        :param source: string, name of the data source, e.g. 'json', 'parquet'.
+        :param source: string, name of the data source, which for now can be 'parquet'.
 
         >>> writer = sdf.writeStream.format('json')
         """

sql/core/src/main/scala/org/apache/spark/sql/streaming/DataStreamWriter.scala

@@ -122,7 +122,7 @@ final class DataStreamWriter[T] private[sql](ds: Dataset[T]) {
 
   /**
    * :: Experimental ::
-   * Specifies the underlying output data source. Built-in options include "parquet", "json", etc.
+   * Specifies the underlying output data source. Built-in options include "parquet" for now.
    *
    * @since 2.0.0
    */