[MLS] AUTH48 for MLS protocol

[Richard Barnes <[e-mail]>]

Hi all,

Good news: The MLS protocol specification has begun the "AUTH48" process,
the last opportunity to make changes before the document is immutably
etched in stone as RFC 9420.

The way AUTH48 usually works is that the authors negotiate changes with the
RFC Editor directly, over email.  But since we've made such productive use
of GitHub, I thought we could bring some more transparency to the process.
I have imported the RFC Editor's proposed XML file (they only do XML, not
Markdown) to the spec repo, and done an edit pass: Changes of theirs that I
accepted are now reflected in the Markdown file; changes that I rejected
are reverted in the "authors' XML" file that we will send back to the RFC
editor.

https://github.com/mlswg/mls-protocol/pull/879

If you could take a look and make sure these edits look OK to you, it would
be helpful.  (Thanks to Brendan for already finding some issues in my
edits!)  I would like to get back to the RFC Editor with our approved
changes by roughly the end of this week, so feedback would be best before
then.

Thanks,
--Richard