[Snac] Stub Router RA flag document updated

[Ted Lemon <[e-mail]>]

The name has changed and I didn't see an announcement go by, so I'm going
to just point it out:

https://datatracker.ietf.org/doc/draft-hui-6man-stub-router-ra-flag/

This document reflects the discussion we had in SNAC yesterday about how to
proceed with the document. The TL;DR is that we took out nearly all the
text describing what stub routers are and just documented the what
non-stub-router implementations need to know about the flag. Which is,
"ignore it." The rest of the document is just allocating the flag. We don't
take a position in whether it's bit 6, 7, or some later bit that will have
to go in the RA flags option.

The reason for doing it this way is that we had a fair amount of text about
what the RA flag is for in the previous version of the document, and this
created a lot of questions in people's minds that had nothing to do with
the actual use of the document. So at this point this document makes it
clear that the ONLY use for this bit is what is documented in
draft-ietf-snac-simple: if you aren't implementing that, all you need to
know is which bit it is, to not set the bit, and to ignore it whether or
not it is set.