net: ip-sysctl: Add link to SCTP IPv4 scoping draft

addr_scope_policy description contains pointer to SCTP IPv4 scoping
draft but not its IETF Datatracker link. Add it.

Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250701031300.19088-6-bagasdotme@gmail.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>