Clarify when the RTP header extension can be set depending on the
value of the `extmap-allow-mixed` option and on whether the header
extension ID is for one-byte or two-bytes extensions.
Bug: b/270541827
Change-Id: I4b939f6862d1f19cbfea11518a1cc1507beb2362
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/294920
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Alessio Bazzica <alessiob@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#39399}
ba3a1e2c26