Ogg Opus Draft: apply some clarifications from derf.
Ogg Opus Draft: Add R128_ALBUM_GAIN.

Patch from Greg Maxwell in response to implementor feedback.
https://www.ietf.org/mail-archive/web/codec/current/msg03053.html
Ogg Opus Draft: Fix a typo.

Thanks to Calvin Walton for pointing this out.
Ogg Opus Draft: Clean up R128 tag descriptions.

Promote the new tag definitions to a separate subsection,
balance tags, improve wording, and document our motivation
for the R128 tags.