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").


For loc, I generally avoid "there is/are" statements and semicolons (though I love semicolons in personal settings!). Maybe "For example, if a configuration (SP: or "course"?) has two experiment groups, each group includes 50% of the students in the course. If the configuration has four experiment groups, each group includes 25% of the students"?