update documents for Table and WriteBuilder

sql/catalyst/src/main/java/org/apache/spark/sql/sources/v2/Table.java

@@ -30,8 +30,8 @@
  * implementation can be a directory on the file system, a topic of Kafka, or a table in the
  * catalog, etc.
  * <p>
- * This interface can mixin the following interfaces to support different operations, like
- * {@code SupportsRead}.
+ * This interface can mixin {@code SupportsRead} and {@code SupportsWrite} to provide data reading
+ * and writing ability.
  * <p>
  * The default implementation of {@link #partitioning()} returns an empty array of partitions, and
  * the default implementation of {@link #properties()} returns an empty map. These should be

sql/core/src/main/java/org/apache/spark/sql/sources/v2/writer/WriteBuilder.java

@@ -70,6 +70,12 @@ default BatchWrite buildForBatch() {
       " does not support batch write");
   }
 
+  /**
+   * Returns a {@link StreamingWrite} to write data to streaming source. By default this method
+   * throws exception, data sources must overwrite this method to provide an implementation, if the
+   * {@link Table} that creates this write returns {@link TableCapability#STREAMING_WRITE} support
+   * in its {@link Table#capabilities()}.
+   */
   default StreamingWrite buildForStreaming() {
     throw new UnsupportedOperationException(getClass().getName() +
       " does not support streaming write");