TYP: Fixup annotations.
Both modified functions may return a MultiIndex; the return annotation has been updated to reflect that.

Added type hint for `names`.
Minor docstring tweak.

- [ ] closes #xxxx
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them

