Rollup merge of #135741 - bardiharborow:std/net/rfc9637, r=Amanieu

Recognise new IPv6 documentation range from IETF RFC 9637

This PR adds the `3fff::/20` range defined by [IETF RFC 9637](https://datatracker.ietf.org/doc/rfc9637/) to those ranges which `Ipv6Addr::is_documentation` recognises as a documentation IP.

See also: https://www.iana.org/assignments/iana-ipv6-special-registry/iana-ipv6-special-registry.xhtml
Unstable tracking issue: #27709

Author: matthiaskrgr