Add note to clearly mark the RFC as rejected

Author: poliorcetics

library/std/src/net/ip.rs

@@ -1265,11 +1265,14 @@ impl Ipv6Addr {
     /// # See also
     ///
     /// - [IETF RFC 4291 section 2.5.6]
+    /// - [RFC 4291 errata 4406] (which has been rejected but provides useful
+    ///   insight)
     /// - [`is_unicast_link_local()`]
     ///
     /// [IETF RFC 4291]: https://tools.ietf.org/html/rfc4291
     /// [IETF RFC 4291 section 2.5.6]: https://tools.ietf.org/html/rfc4291#section-2.5.6
     /// [`true`]: ../../std/primitive.bool.html
+    /// [RFC 4291 errata 4406]: https://www.rfc-editor.org/errata/eid4406
     /// [`is_unicast_link_local()`]: ../../std/net/struct.Ipv6Addr.html#method.is_unicast_link_local
     pub fn is_unicast_link_local_strict(&self) -> bool {
         (self.segments()[0] & 0xffff) == 0xfe80
@@ -1321,9 +1324,12 @@ impl Ipv6Addr {
     /// # See also
     ///
     /// - [IETF RFC 4291 section 2.4]
+    /// - [RFC 4291 errata 4406] (which has been rejected but provides useful
+    ///   insight)
     ///
     /// [IETF RFC 4291 section 2.4]: https://tools.ietf.org/html/rfc4291#section-2.4
     /// [`true`]: ../../std/primitive.bool.html
+    /// [RFC 4291 errata 4406]: https://www.rfc-editor.org/errata/eid4406
     /// [`is_unicast_link_local_strict()`]: ../../std/net/struct.Ipv6Addr.html#method.is_unicast_link_local_strict
     pub fn is_unicast_link_local(&self) -> bool {
         (self.segments()[0] & 0xffc0) == 0xfe80