Info for moderators

@explorerleslie , @cahrens , @gwprice , @srpearce , please take a look! HTML version here: http://draft-cohorts.readthedocs.org/en/latest/cohorts/cohorts_discussions.html
This is not quite accurate; you can only choose to make a post visible to a specific cohort if the topic is divided. If the user selects a unified topic, then the "Visible to" selector will show "All Groups" and become disabled.