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.
Just from the perspective of that it's a pain in the butt to enter all those individual email addresses, I would be surprised if people use all manual assignment in a class any larger than 1000. 

However, do note that even if the course team only creates manual cohort groups, there is still a "default cohort group" that catches anyone that the course staff forgets to add to a cohort...this is so a student who is missed isn't totally left out of the conversation :)