Added docs for the change of behavior of isin
This is to close issue #38781, I added documentation to explain the change of behavior of the isin function.

- [x] closes #38781 
- [ ] 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
- [x] whatsnew entry

Hello @omarafifii! 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 2021-01-16 13:11:57 UTC I added the docs to the isin in the series.py file. Should I also do the same for the one in frame.py? we may also want to add an entry in the release notes for 1.2 about this change and also a note in the 1.2.1 release notes (other section) that the release notes for 1.2 are updated.  thanks @omarafifii and @simonjayhawkins  @meeseeksdev backport 1.2.x Something went wrong ... Please have  a look at my logs.