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
