sdpFmtpLine prose is vague about which SDP to use

[jan-ivar]

webrtc-pc sdpFmtpLine parameter says: "... For an RTCRtpSender, these parameters come from the remote description, and for an RTCRtpReceiver, they come from the local description."

This is vague. We should update this to say: "For an RTCRtpSender, these parameters come from the current remote description, and for an RTCRtpReceiver, they come from the (potentially pending) local description."

This is because:

• sender getParameters gets it from [[SendCodecs]] which is set in the answer or pranswer.
• receiver getParameters gets it from [[ReceiveCodecs]] which is set in SLD.