DOC: GH5456 Adding workaround info on NA / NaT handling for groupby
- [x] closes #5456
- [NA] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [NA] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [NA] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

Took a stab at this by parsing through the initial issue and related topics - let me know if the intent was different, happy to edit.

Good point @phofl. Looks we also have an example in the groupby docstring demonstrating this behavior so I don't think this workaround explanation is needed. 

Thanks for the PR @aamnv but going to close since I think this old issue is not actionable anymore, but feel free to address any of the other `good first issue` issues!