DOC: Added filtering example to groupby doc
closes https://github.com/pydata/pandas/issues/4630

can you rebase and squash these down?

see here: https://github.com/pydata/pandas/wiki/Using-Git
 cherry-picked https://github.com/pydata/pandas/commit/eb49283409b04ee481dfa81a49d78c1c437b73f6

thanks!
