DOC: .groupby() aligns Series, accepts ndarray
 - [x] closes https://github.com/pandas-dev/pandas/issues/15244
 - [ ] tests added / passed
 - [x] passes ``git diff upstream/master --name-only -- '*.py' | flake8 --diff``
 - [ ] whatsnew entry

Multiple tests already group on ndarrays, so I suppose this API is covered? yes thanks @kernc (and ignore the appveyor failure). thanks!