Rollup merge of rust-lang#75150 - nanpuyue:deprecate_to_ipv6_compatible, r=LukasKalbertodt

Add a note for Ipv4Addr::to_ipv6_compatible

Previous discussion: rust-lang#75019

> I think adding a comment saying "This isn't typically the method you want; these addresses don't typically function on modern systems. Use `to_ipv6_mapped` instead." would be a good first step, whether this method gets marked as deprecated or not.

_Originally posted by @joshtriplett in rust-lang#75150 (comment)

Author: Dylan-DPC

library/std/src/net/ip.rs

@@ -745,6 +745,9 @@ impl Ipv4Addr {
     ///
     /// a.b.c.d becomes ::a.b.c.d
     ///
+    /// This isn't typically the method you want; these addresses don't typically
+    /// function on modern systems. Use `to_ipv6_mapped` instead.
+    ///
     /// [`IPv6` address]: Ipv6Addr
     ///
     /// # Examples