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.
You might repeat here that when you use cohorts, you can create two types of discussions: content-specific and course-wide. (I know that's implied in the bullet points above, but someone may skip over that section, and I don't think it would hurt to say it specifically.)