DOC: Add notes section to .isin() docs
Add a Notes section to the .isin() method docs to clarify that we use __iter__ over __contains__ when checking each element in the `values` collection passed as argument. This clarification was requested because `values` can be a dict, which suggests that under the hood isin() uses __contains__ rather than __iter__, which is not the case and might confuse some users.

- [x] closes #59041 
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit)

Output of the docs checker script, same as before adding the doc change:
```
1 Errors found for `pandas.DataFrame.isin`:
	ES01	No extended summary found
```

![image](https://github.com/pandas-dev/pandas/assets/24620565/81d3d626-b2bf-4bee-b542-3c6147572d6d)

Thanks @AnaDenisa 