Update README.

README.md

@@ -38,8 +38,9 @@ static pointer create_dummy();
 
 // Function to perform actual network send()/recv() I/O:
 // (note: if all you need is to recv()/dispatch() messages, then timeout can be
-// used to block until a message arrives)
-void poll(int timeout = 0);
+// used to block until a message arrives. By default, when timeout is 0, poll()
+// will not block at all.)
+void poll(int timeout = 0); // timeout in milliseconds
 
 // Receive a message, and pass it to callable(). Really, this just looks at
 // a buffer (filled up by poll()) and decodes any messages in the buffer.