DOC: Improve groupby().ngroup() explanation for missing groups
The exact prose and examples are open to improvements if you have ideas.

I figure this didn't require a separate issue, please let me know if I'm missing some step in the process.
@datapythonista @mroeschke @rhshadrach this is ready for re-review! Sorry for the tag if this is already in your queue :)

That failing test looks unrelated to this change. Thanks @NickCrews