Adding to the function docs that groupby.transform() function parameter can be a string
Docs change: Adding to the function docs that groupby.transform() function parameter can be a string

- [ ] closes #49961 
- [ ] [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
- [ ] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

Thanks @seanjedi  @WillAyd I feel examples should be added for both, DataFrameGroupBy.transform() and SeriesGroupBy.transform() with strings