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.
Ditto what Leslie said-- could have any number of auto groups in addition to the manual groups. But if the course only wants a single automatic cohort, they can just rely on the default one and don't have to explicitly specify an automatic cohort.

As far as Leslie's second comment, in reality cohorting with manual cohorts is ALWAYS a hybrid approach, because a default cohort will be created for you if any user visits the discussion forums and is not already put into a cohort. So it is always true that people being manually assigned need to have the assignment done before they first visit the discussion forums.