DOC: Improve the docstring of pd.Index.contains
- [x] closes #20211 
- [ ] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

This PR updates the previous PR #20211, based on reviews and docstring standards. It was worked upon with the help of @datapythonista

Kindly review, I will make all the necessary changes to it.