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. 
Ah, @lamagnifica  I see this separation now between student and instructor enrollment events... Technically, these are the same events & there is no way to distinguish them by name alone. Should they be distinguished in the docs like this?