docs: event emitter behavior notice

Updated documentation as per the issue below:
nodejs/node-v0.x-archive#25466

Event listeners can alter parts of the passed object, in some
circumstances the changes are passed to the next listeners
due to pass by reference. This is documentation of that behavior.

PR-URL: nodejs/node-v0.x-archive#25467
Reviewed-By: jasnell - James M Snell <jasnell@gmail.com>

doc/api/events.markdown

@@ -118,6 +118,10 @@ Execute each of the listeners in order with the supplied arguments.
 
 Returns `true` if event had listeners, `false` otherwise.
 
+**Note:** Objects are passed by reference, meaning that the same object can be 
+passed to multiple listeners. If a listener needs to modify a shared object, a 
+copy of the object should be made.
+
 
 ### Class Method: EventEmitter.listenerCount(emitter, event)