Better documentation of ended event

[eladalon1983]

MediaStreamTrack.ended reads:

The MediaStreamTrack object's source will no longer provide any data, either because the user revoked the permissions, or because the source device has been ejected, or because the remote peer permanently stopped sending data.

This fails to mention the caveat here:

When a MediaStreamTrack track ends for any reason other than the stop() method being invoked
...
4. Fire an event named ended at the object.

Possibly we could extend the desciption: "... Note that if the application itself stopped the track, by calling the stop() method, then this event will not be fired."