net: document that port 0 picks a random port (#4386)

tokio/src/net/udp.rs

@@ -128,6 +128,10 @@ impl UdpSocket {
     /// This function will create a new UDP socket and attempt to bind it to
     /// the `addr` provided.
     ///
+    /// Binding with a port number of 0 will request that the OS assigns a port
+    /// to this listener. The port allocated can be queried via the `local_addr`
+    /// method.
+    ///
     /// # Example
     ///
     /// ```no_run