TST: Add docstrings to arithmetic fixtures
Relates to #19159

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

Hello @dalgarno! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:



There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2019-11-12 21:52:27 UTC lgtm. side issue we should rename these to maybe ``one_value``and so on and move to pandas/conftest.py (but for another PR); also I believe we have *multiple* copies of these fixtures in various locations. @WillAyd can you approve if you're happy? Thanks @dalgarno 