DOC: Cleaned docstrings in core/indexers.py
- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

@MomIsBestFriend can you rebase rebased @jbrockmendel  Thank you @jorisvandenbossche  (Will merge when green) Thanks @MomIsBestFriend !