DOC: GH10414 Missing example in NA values in GroupBy
closes #10414 

@harshnisar see the contributing docs [here](http://pandas.pydata.org/pandas-docs/stable/contributing.html)

pls pull current master and rebase. I know its only a trivial change, but this is in conflict as master changed a tiny bit in this section a couple of days ago.
 @jreback

Done :)
 Looks good now! @harshnisar Last question: can you squash the three commits into one? (see end of this section: http://pandas.pydata.org/pandas-docs/stable/contributing.html#committing-your-code)
 @jorisvandenbossche  I know by now, this pull request is more of an effort sink for you than a contribution by me. Won't happen from next pulls.
 @harshnisar no, no problem! And thanks!
