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
"student" rather than "user"?