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. 
@lamagnifica  Any reason to explicitly point out either here or in the description of the event that user_id might not be the same as event.user_id?
I.e. that the top-level user_id is the user initiating the enrollment request, while event.user_id is the user/student getting enrolled? 

That is,
the current per-field description is very clear; however, would a short sentence connecting the dots be useful?