Make it clear that to_ipv4 returns an IPv4 address for the IPv6 loopback

Author: tbu-

library/std/src/net/ip.rs

@@ -138,6 +138,7 @@ 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`.
 ///
 /// [IETF RFC 4291 Section 2.5.5.2]: https://datatracker.ietf.org/doc/html/rfc4291#section-2.5.5.2
 ///
@@ -1673,7 +1674,9 @@ impl Ipv6Addr {
     /// or an [IPv4-mapped] address as defined in [IETF RFC 4291 section 2.5.5.2],
     /// otherwise returns [`None`].
     ///
-    /// `::a.b.c.d` and `::ffff:a.b.c.d` become `a.b.c.d`
+    /// Note that this will return an [`IPv4` address] for the IPv6 loopback address `::1`.
+    ///
+    /// `::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`.
     ///
     /// [`IPv4` address]: Ipv4Addr