DOC: Clarify in pandas.DataFrame.drop that a copy is returned when inplace = False.
Clarify in `pandas.DataFrame.drop` that a copy is returned when `inplace = False`.

- [x] closes #33451  
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
