DOC: update the Series.str.join docstring
Checklist for the pandas documentation sprint (ignore this if you are doing an unrelated PR):

- [x] PR title is "DOC: update the docstring"
- [x] The validation script passes: scripts/validate_docstrings.py <your-function-or-method>
- [x] The PEP8 style check passes: git diff upstream/master -u -- "*.py" | flake8 --diff
- [x] The html version looks good: python doc/make.py --single <your-function-or-method>
- [x] It has been proofread on language by another sprint participant