doc: Add callback parameter to dgram socket.bind()

Also, describe more details of bind().

doc/api/dgram.markdown

@@ -127,13 +127,21 @@ informing the source that the data did not reach its intended recipient).
 
 * `port` Integer
 * `address` String, Optional
-* `callback` Function, Optional
+* `callback` Function with no parameters, Optional. Callback when
+  binding is done.
 
-For UDP sockets, listen for datagrams on a named `port` and optional `address`.
-If `address` is not specified, the OS will try to listen on all addresses.
+For UDP sockets, listen for datagrams on a named `port` and optional
+`address`. If `address` is not specified, the OS will try to listen on
+all addresses.  After binding is done, an "listening" event is emitted
+and the `callback`(if specified) is called. Specifying both an
+"listening" event listener and `callback` is not harmful but not very
+useful.
 
-The `callback` argument, if provided, is added as a one-shot `'listening'`
-event listener.
+A bound datagram socket keeps the node process running to receive
+datagrams.
+
+If binding fails, an "error" event is generated. In rare case (e.g.
+binding a closed socket), an `Error` may be thrown by this method.
 
 Example of a UDP server listening on port 41234:
 
@@ -141,6 +149,11 @@ Example of a UDP server listening on port 41234:
 
     var server = dgram.createSocket("udp4");
 
+    server.on("error", function (err) {
+      console.log("server error:\n" + err.stack);
+      server.close();
+    });
+
     server.on("message", function (msg, rinfo) {
       console.log("server got: " + msg + " from " +
         rinfo.address + ":" + rinfo.port);