Cohorted courseware: Reword cohorts and content experiment overview/config in preparation

Preparation for cohorted courseware doc: separate out the conceptual and configuration content that applies to cohorted content groups vs content experiments; update terminology for the redesigned group configurations page ("experiment groups").


I think this sentence is easier to comprehend without "that is inside a group configuration currently being used". That is, 
"However, before you modify any other characteristics of a group in a running course, review..."