Recommend Ipv6Addr::to_ipv4_mapped over Ipv6::to_ipv4 once it's stable

[tbu-]

CC #27709 (tracking issue for Ipv6Addr::to_ipv4_mapped)
CC #96649 (PR that tried to add this change first)

diff --git a/library/std/src/net/ip.rs b/library/std/src/net/ip.rs
index f629a1a0f99..dfec52b503a 100644
--- a/library/std/src/net/ip.rs
+++ b/library/std/src/net/ip.rs
@@ -138,7 +138,8 @@ pub struct Ipv4Addr {
 ///
 /// To convert from an IPv4 address to an IPv4-mapped IPv6 address, use [`Ipv4Addr::to_ipv6_mapped`].
 /// Use [`Ipv6Addr::to_ipv4`] to convert an IPv4-mapped IPv6 address to the canonical IPv4 address.
-/// Note that this will also convert the IPv6 loopback address `::1` to `0.0.0.1`.
+/// Note that this will also convert the IPv6 loopback address `::1` to `0.0.0.1`. Use
+/// [`Ipv6Addr::to_ipv4_mapped`] to avoid this.
 ///
 /// [IETF RFC 4291 Section 2.5.5.2]: https://datatracker.ietf.org/doc/html/rfc4291#section-2.5.5.2
 ///
@@ -1674,7 +1675,8 @@ pub const fn to_ipv4_mapped(&self) -> Option<Ipv4Addr> {
     /// or an [IPv4-mapped] address as defined in [IETF RFC 4291 section 2.5.5.2],
     /// otherwise returns [`None`].
     ///
-    /// Note that this will return an [`IPv4` address] for the IPv6 loopback address `::1`.
+    /// Note that this will return an [`IPv4` address] for the IPv6 loopback address `::1`. Use
+    /// [`Ipv6Addr::to_ipv4_mapped`] to avoid this.
     ///
     /// `::a.b.c.d` and `::ffff:a.b.c.d` become `a.b.c.d`. `::1` becomes `0.0.0.1`.
     /// All addresses *not* starting with either all zeroes or `::ffff` will return `None`.