Document the requirement for the send_async method

See apache#184

It should be documented that the callback should not throw any exception
when passed to `send_async`.

Author: BewareMyPower

pulsar/__init__.py

@@ -1237,6 +1237,8 @@ def send_async(self, content, callback,
         --------
 
         The ``callback`` will be invoked once the message has been acknowledged by the broker.
+        Users are responsible to handle the exception inside the callback. If any exception was
+        thrown from the callback, the process would terminate immediately.
 
         .. code-block:: python