Correct deprecated is_global IPv6 documentation

This method does currently not return false for the `site_local` unicast address space. The documentation of the `is_unicast_global` method on lines 1352 - 1382 suggests that this is intentional as the site-local prefix must no longer be supported in new implementations, thus the documentation can safely be updated to reflect that information.
If not so,  either the `is_unicast_global` method should be updated to exclude the unicast site-local address space, or the `is_global` method itself.

src/libstd/net/ip.rs

@@ -1130,7 +1130,7 @@ impl Ipv6Addr {
     /// The following return [`false`]:
     ///
     /// - the loopback address
-    /// - link-local, site-local, and unique local unicast addresses
+    /// - link-local, and unique local unicast addresses
     /// - interface-, link-, realm-, admin- and site-local multicast addresses
     ///
     /// [`true`]: ../../std/primitive.bool.html