doc: document return values of EventEmitter methods

doc/api/events.markdown

@@ -39,6 +39,8 @@ Adds a listener to the end of the listeners array for the specified event.
       console.log('someone connected!');
     });
 
+Returns emitter, so calls can be chained.
+
 ### emitter.once(event, listener)
 
 Adds a **one time** listener for the event. This listener is
@@ -49,6 +51,8 @@ it is removed.
       console.log('Ah, we have our first user!');
     });
 
+Returns emitter, so calls can be chained.
+
 ### emitter.removeListener(event, listener)
 
 Remove a listener from the listener array for the specified event.
@@ -61,11 +65,13 @@ Remove a listener from the listener array for the specified event.
     // ...
     server.removeListener('connection', callback);
 
+Returns emitter, so calls can be chained.
 
 ### emitter.removeAllListeners([event])
 
 Removes all listeners, or those of the specified event.
 
+Returns emitter, so calls can be chained.
 
 ### emitter.setMaxListeners(n)
 
@@ -89,6 +95,8 @@ Returns an array of listeners for the specified event.
 
 Execute each of the listeners in order with the supplied arguments.
 
+Returns `true` if event had listeners, `false` otherwise.
+
 
 ### Class Method: EventEmitter.listenerCount(emitter, event)