Ahodges/doc/cohorts

@explorerleslie , @nasthagiri , @andy-armstrong , @gwprice , @jimabramson , @dan-f , @cahrens, @srpearce  , @WatsonEmily , @mhoeber 
The cohorts/cohort_config and cohorts/cohorts_overview files and images are pretty solid. Questions occasionally in comments.
I'll work on the section describing moderating discussions in a cohorted universe next.
Yeah, I agree with Zach...we should be framing this more generally around naming conventions. You could either flag discussion topics that are public or private, depending on what is most appropriate for your course. 

We aren't targeting this functionality toward courses that want completely private discussions (no public discussions), so we don't really want to sell that angle.