DOC: Updated docstring for Series.str.cat (pandas-dev#35556)
- [X] closes #35556
- [X] tests added / passed
- [X] passes `black pandas`
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

I have updated the docstring for `Series.str.cat` to make the issue in #35556 more explicit to the user.