- Adding information about RTCP and RTP header extensions. - Renaming flexfec_payload_type -> payload_type and flexfec_ssrc -> remote_ssrc. BUG=webrtc:5654 R=stefan@webrtc.org, philipel@webrtc.org Review-Url: https://codereview.webrtc.org/2542413002 Cr-Commit-Position: refs/heads/master@{#15477}