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. 
@mulby We took the following comment in the gist:

```
We typically use "_" as a word separator in field names so this should either be a nested dict (like I described) or at least "app_name".
```

as an invitation to go with the `app_name` alternative.