[webrtc-pc] Pull Request: RTP/RTCP mux -> RTCP mux

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

== RTP/RTCP mux -> RTCP mux ==
replace single use of RTP/RTCP mux with just RTCP mux for consitency with
other uses

We also don't define "RTCP mux" anywhere and don't have a reference to RFC 5763. Should we?

See https://github.com/w3c/webrtc-pc/pull/1247