[SPARK-16084][SQL] Minor comments update for "DESCRIBE" table

## What changes were proposed in this pull request?

1. FORMATTED is actually supported, but partition is not supported;
2. Remove parenthesis as it is not necessary just like anywhere else.

## How was this patch tested?

Minor issue. I do not think it needs a test case!

Author: bomeng <bmeng@us.ibm.com>

Closes #13791 from bomeng/SPARK-16084.