New section for video events emitted by mobile app, fixes DOC-1219

@nasthagiri , @stroilova , please verify the accuracy of all changed lines, particularly the new section for mobile video events and the existing section for browser video events.
@mulby FYI
@catong , @mhoeber , and @srpearce , events are expected to be in packages on Thursday. 
Sometimes these activities are completed by an instructor. Should the verb here be 'enrollment status changes for a student', including students enrolling in a course or unenrolling (i.e. removing a course from their dashboard), intsructors manually enrolling or unenrolling students, either individually or in batches, from the Instructor Dashboard, or other manual interventions.
this is a bit of a mouthful! 
What do you think @lamagnifica -- what is the best way to transmit the use case info to research teams here ?