net: display net requirement for net::UdpSocket in docs (#6938)

Author: jofas

tokio/src/net/mod.rs

@@ -46,6 +46,7 @@ cfg_net! {
         pub use tcp::socket::TcpSocket;
 
         mod udp;
+        #[doc(inline)]
         pub use udp::UdpSocket;
     }
 }