[SPARK-15973][PYSPARK] Fix GroupedData Documentation

*This contribution is my original work and that I license the work to the project under the project's open source license.*

## What changes were proposed in this pull request?

Documentation updates to PySpark's GroupedData

## How was this patch tested?

Manual Tests

Author: Josh Howes <josh.howes@gmail.com>
Author: Josh Howes <josh.howes@maxpoint.com>

Closes #13724 from josh-howes/bugfix/SPARK-15973.