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
Is it clear enough that this description includes adding students via a csv upload? If not too wordy/specific, perhaps add "either manually or using a csv file"?