DOC: Clarify DataFrame.combine_first and Series.combine_first
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew entry

Hello @HeidiNeuhaeuser! 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 2021-03-07 19:40:27 UTC thanks @HeidiNeuhaeuser if you'd like to make a shared doc-string as suggested by @MarcoGorelli pls open a new PR