spec walkthrough and edits

comments to the changes
1. "channel" is not exposed or controlled by any API in this spec
2. MediaStream doesn't have "input" or "output" concept directly. 
3. Both MediaStream and MediaStreamTrack can be consumed by other objects, for example, MediaStreamTrack by ORTC RTCRtpSender object.
4. We should remove the "local file" scenario.  It can go to another spec if necessary.  It seems most scenario can be covered with MediaStream from MediaElement (when that spec is ready).