std::net: add warning in Ipv4addr::is_reserved() documentation

See @the8472 comment's on Github:
https://github.com/rust-lang/rust/pull/60145#issuecomment-485424229

> I don't think is_reserved including ranges marked for future use is
> a good idea since those future uses may be realized at at some point
> and then old software with is_reserved filters may have false
> positives. This is not a hypothetical concern, such issues have been
> encountered before when IANA assigned previously reserved /8 address
> blocks.

src/libstd/net/ip.rs

@@ -677,6 +677,13 @@ impl Ipv4Addr {
     /// [IETF RFC 1112]: https://tools.ietf.org/html/rfc1112
     /// [`true`]: ../../std/primitive.bool.html
     ///
+    /// # Warning
+    ///
+    /// As IANA assigns new addresses, this method will be
+    /// updated. This may result in non-reserved addresses being
+    /// treated as reserved in code that relies on an outdated version
+    /// of this method.
+    ///
     /// # Examples
     ///
     /// ```