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. 
I find the "this event results" construction awkward, and prefer the original with tweaks.
Something like, "For example, when a student clicks Enroll for a course on the edx.org site, this event is emitted."