explain that ShutdownListeners run in the connection's thread

Author: Tim Watson

src/com/rabbitmq/client/ShutdownListener.java

@@ -23,6 +23,9 @@
  * A ShutdownListener receives information about the shutdown of connections and
  * channels. Note that when a connection is shut down, its associated channels are also
  * considered shut down and their ShutdownListeners will be notified (with the same cause).
+ * Because of this, and the fact that channel ShutdownListeners execute in the connection's
+ * thread, attempting to make blocking calls on a connection inside the listener will
+ * lead to deadlock.
  *
  * @see ShutdownNotifier
  * @see ShutdownSignalException