[SPARK-10380][SQL] Fix confusing documentation examples for astype/drop_duplicates.

## What changes were proposed in this pull request?
We have seen users getting confused by the documentation for astype and drop_duplicates, because the examples in them do not use these functions (but do uses their aliases). This patch simply removes all examples for these functions, and say that they are aliases.

## How was this patch tested?
Existing PySpark unit tests.

Closes #11543.

Author: Reynold Xin <rxin@databricks.com>

Closes #11698 from rxin/SPARK-10380.