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 called it out here -> https://gist.github.com/nasthagiri/58ec88c1a92e6641dd0e

Looking back at it, it looks like my comment was never actually acknowledged. Is it too late to do this? We are going to have to end up supporting this for a very long time...

@stroilova - my understanding is that there is a context->client->app->name field, however, we have very little control over it (it is part of the android app metadata). At event emission time it is much easier to control fields in the root of the context dictionary as opposed to the client sub-dict (which is controlled by segment.io).