DOC: updating the Series.str.cat docstring
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`


We have run ``validate_docstrings.py`` and gotten the following error:

```
Errors found:
  Errors in parameters section
       Parameter "join" description should finish with "."
```
This is nothing to do with our PR.  We're great. I think the error is because the "." is followed by "`other`" @datapythonista may be you can varify Pretty sure that error is from the `.. versionadded:: `. We need to update the script to not complain when there isn't an ending period for those. Thanks!