[SPARK-25328][PYTHON] Add an example for having two columns as the grouping key in group aggregate pandas UDF
[SPARK-25328][PYTHON] Add an example for having two columns as the grouping key in group aggregate pandas UDF

## What changes were proposed in this pull request?

This PR proposes to add another example for multiple grouping key in group aggregate pandas UDF since this feature could make users still confused.

## How was this patch tested?

Manually tested and documentation built.

Closes #22329 from HyukjinKwon/SPARK-25328.

Authored-by: hyukjinkwon <2680965ad5aae29275c8fb10150cb10f5554e78b@apache.org>
Signed-off-by: Bryan Cutler <060dd14c05a8ffa8f201883ef57d0676b7c9c04a@gmail.com>

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