[webrtc-pc] RTP Media API section should describe how tracks for senders/receivers relate to each other.

taylor-b has just created a new issue for https://github.com/w3c/webrtc-pc:

== RTP Media API section should describe how tracks for senders/receivers relate to each other. ==
Previously, there was a 1:1 mapping between tracks added to a PeerConnection on one side and received on the other side.

The track IDs also matched (https://github.com/w3c/webrtc-pc/issues/1128).

And when you called `removeTrack`, the track on the other side would be removed and ended (https://github.com/w3c/webrtc-pc/issues/1161).

None of these things are true any more, due to the API changes to allow for "early media", transceivers, `replaceTrack`, etc.

The fundamental difference is that tracks on senders and receivers are not symmetrical. A track attached to a sender represents "a source of media being sent", whereas a track attached to a receiver represents "a stream of media received from an `RTCRtpSender` on the remote side`.

This clearly has been a source of confusion, and it's not explained in any unified way. So I'd suggest adding a paragraph to the RTP Media API section explaining this model.

Please view or discuss this issue at https://github.com/w3c/webrtc-pc/issues/1169 using your GitHub account