Revisions to descriptions of cohort events

@catong these are the events that are emitted when things relating to cohorts happen. Basically, I wanted to get rid of "manually" in relation to course teams adding students to cohorts. But I took the opportunity to remove the Latin, inlcude xrefs to the B&R guide, and otherwise polish up these descriptions. No rush.
@stroilova, FYI
Thank you @lamagnifica -- did we end up specifying explicitly both scenarios:
- instructors manually adding students to a cohort (I'm assuming using a UI in the Instructor Dashboard or Studio?) , and
- instructors uploading a CSV of students with cohort assignments

By the way, do the events distinguish these two scenarios, and should we run the final language by the implementing team?

FYI : @mulby  , @gwprice ( ? ) , @cahrens ( ? )