Consequences of changing configuration

@cahrens , @explorerleslie , @gwprice , @zhkramer , @srpearce : last big chunk, let me know what I've gotten wrong and what I've forgotten!
This makes it sound like it will only assign new users to newly defined cohort groups. It will assign new users to any of the cohort groups currently listed in auto_cohort_groups. This may include some "original cohort groups", and some newly defined ones.

For example, assume auto_cohort_groups used to be [a,b]. Then you changed it to [a,c]. New students are randomly assigned to auto cohort groups a and c. Group b is now a "manual" group, and no new students will automatically be added to it.

I'm not sure how much rewording is worthwhile, but for example "The original cohort group or groups are converted into manual cohort groups" could be rewritten as "Previously defined auto cohort groups that are no longer included in auto_cohort_groups will be converted to manual cohort groups".