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
@stroilova , I just got these comments (several days after the PR was closed and merged and docs published). 
For your doc question, yes, both paths to adding students to a specific cohort are explicitly called out in teh documentation (lines 3320-3322 immediately below, or http://edx.readthedocs.org/projects/devdata/en/latest/internal_data_formats/tracking_logs.html#edx-cohort-user-added).
I do not believe that the T&L team changed the eventing in any way when they expanded the feature to allow course teams to semi-automate assignment of students to specific cohorts by uploading a CSV. Please open a JIRA ticket for the discovery or potential addition to the events. I'm closing this PR now, which was solely to remove "manually" etc. as stated in the description.