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.
the light dawned while I was eating breakfast...  the issue with how I phrased this isn't just here, it's throughout, and needs to be replaced. I'll hipchat you an idea for a better definition of an "open" discussion topic, to make sure I'm on the right track