[SPARK-18493] Add missing python APIs: withWatermark and checkpoint to dataframe

## What changes were proposed in this pull request?

This PR adds two of the newly added methods of `Dataset`s to Python:
`withWatermark` and `checkpoint`

## How was this patch tested?

Doc tests

Author: Burak Yavuz <brkyvz@gmail.com>

Closes #15921 from brkyvz/py-watermark.