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. 
Wait, I thought we were going to move this into an "application" dictionary along with "component", are they both still at the top level of the context?

@nasthagiri @aleffert