DOC: Improve documentation for Index.where
- [x] closes https://github.com/pandanistas/pandanistas_sprint_jakarta2020/issues/15
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Thanks for the work on this @laymonage 