`Ipv6Addr::is_documentation` should be expanded to include RFC 9637's `3ffff::/20`

[jrose-signal]

The unstable Ipv6Addr::is_documentation (#27709) is documented, matching the implementation, as

Returns true if this is an address reserved for documentation (2001:db8::/32).

This property is defined in IETF RFC 3849.

However, RFC 9637 from Aug 2024 added 3fff::/20 as a documentation space as well, and thus this API should be updated.

This does mean two programs may disagree on whether an address is_documentation based on which version of the stdlib they use…but I think that's inevitable with anything based on IETF standards. (In some cases it might make sense to explicitly pin the library to an earlier version of the standard, but I don't think that's necessary here.)

[the8472]

This does mean two programs may disagree on whether an address is_documentation based on which version of the stdlib they use

Even if that were a concern (I don't think it is), the feature is unstable so we're free to change it anyway.