DOC: update shared_docs to clarify groupby with lists
- [x] closes #44333

Following @rhshadrach 's and @asishm 's suggestion, this PR modifies groupby docstring.
Hello @arushi-08! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:



There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2021-11-24 05:22:52 UTC thanks @arushi-08 