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. 
@aleffert I would prefer not to include iOS primarily because no events will include that value yet, and also because I think that a single example value is sufficient and less likely to require updates over time.