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.
@lamagnifica Yeah, my bad. Students will see every discussion topic, but they don't see every post within the topic. The topic/post terminology still trips me up and I'm the product manager on the feature... So I don't think you need to change anything here.