doc: dgram: add v0.10 bind() behavior note

dgram.Socket#bind() is always asynchronous now. Add a note at the top
of the documentation that explains how to upgrade.

Fixes #4944.

doc/api/dgram.markdown

@@ -6,6 +6,21 @@
 
 Datagram sockets are available through `require('dgram')`.
 
+Important note: the behavior of `dgram.Socket#bind()` has changed in v0.10
+and is always asynchronous now.  If you have code that looks like this:
+
+    var s = dgram.createSocket('udp4');
+    s.bind(1234);
+    s.addMembership('224.0.0.114');
+
+You have to change it to this:
+
+    var s = dgram.createSocket('udp4');
+    s.bind(1234, function() {
+      s.addMembership('224.0.0.114');
+    });
+
+
 ## dgram.createSocket(type, [callback])
 
 * `type` String. Either 'udp4' or 'udp6'