[3.13] gh-87245: Improve IPv6Address.ipv4_mapped documentation (GH-92572) (#149361)

gh-87245: Improve IPv6Address.ipv4_mapped documentation (GH-92572)

Avoid the phrasing ‘starting with ::FFFF/96’, which is confusing since
it seems to mix a prefix and a range. Instead, make it clear what the
actual range is, and refer to the relevant RFC.

Closes GH-87245.
(cherry picked from commit 7aedd0a6c6ae3b5cdd601e84411e41c56f8d00b9)

Co-authored-by: wouter bolsterlee <wouter@bolsterl.ee>