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!
I think it would make more sense to call this subsection "Changing the auto_cohort_groups setting" and describe the outcome of changing the setting more generally. We should also include the fact that the system does not do any kind of rebalancing. Whenever you make a change to the setting, future random assignments will be uniformly distributed among the listed groups, regardless of the previous size of each group.