Improve return description in `droplevel` docstring
Use https://pandas-docs.github.io/pandas-docs-travis/reference/api/pandas.DataFrame.dropna.html#pandas.DataFrame.dropna
docstring as a\ template.

MINOR DOCUMENTATION CHANGE ONLY (ignored below)

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
