DOC: .get_slice_bound in MultiIndex needs documentation.
- [x] closes #29967
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

I'm not good at English. So if there are wrong sentences or awkward sentences, please let me know.

Hello @proost! 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 2019-12-23 09:43:50 UTC thanks @proost 