[webrtc-pc] Pull Request: Adding note about the relationship between tracks of senders/receivers.

taylor-b has just submitted a new pull request for https://github.com/w3c/webrtc-pc:

== Adding note about the relationship between tracks of senders/receivers. ==
Fixes #1169.

Clarifies that there's not a 1:1 relationship between tracks sent by
one PeerConnection and received by the other (due to `replaceTrack`),
and the IDs are not guaranteed to match. So it's better to think of
a 1:1 relationship between senders on one side and receivers on the
other side, rather than tracks.

See https://github.com/w3c/webrtc-pc/pull/1911