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.
