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. 
The apps aren't sending ``load_video`` directly. I'm assuming that's something that happens when we process what we receive from segment, but I figured I should double check. Is that correct?