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
Preferred: focus on the actions by course team/system vs the users who are being added.  "Members of the course team can add users to cohort groups. The system automatically adds..."