RTP draft edits (no normative changes).

This is the result of an editing pass for clarity and consistency.
RTP draft edits (normative changes).

Here are my proposals to resolve a few issues with the current
 draft.
See the corresponding message to the list for details.
Merge remote-tracking branch 'origin/master' into derf_rtp_edits

Conflicts:
	doc/draft-ietf-payload-rtp-opus.xml