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").


Instead of using "or" to join the two options, I'd recast this:

... used by that experiment in two ways.

* In a unit...
* At the top of...