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 would rephrase this. It APPEARS that they have saved because you get the confirmation message with the checkmark. But in reality, your attempted changes are replaced with what the previous value was.

So maybe "Entries that do not contain all of the required punctuation characters will be replaced by the previous value for Cohort Configuration upon save, and no warning is presented."