DOC: remove note about groupby.apply side-effect
The described behavior has been fixed in 0.25

- [x] closes #44318
- [ ] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [x] whatsnew entry, _comment:_ wouldn't say this is applicable here

thanks @k-dominik  thx for the quick responses @jreback!