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.
I don't believe that is true, @lamagnifica. If there is a syntax error in what you save (which is the case when there is no comma), the save appears to work, but the value for the field reverts to what was there before. I just tried your case of not putting in a coma, and the "cohorted_discussions" disappear upon save (if they previously were not specified).