DOC: make return type documentation of series methods consistent #35409
- [x] closes #35409 
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
Fix docstrings associated with Series methods for now.
Please review.

Once it passes review, I'll start working on other docstring fixes in the same way.
 would love to get a hacktoberfest tag if possible. #36837 cc @simonjayhawkins ok with the text? Thanks @junjunjunk! Very nice.