[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.