Added documentation to signify that listeners are called in the order…

… they were added, see phetsims/graphing-lines#109

js/Emitter.js

@@ -108,7 +108,7 @@ define( require => {
     }
 
     /**
-     * Adds a listener
+     * Adds a listener.  When emitting, the listeners are called in the order they were added.
      * @param {function} listener
      * @public
      */
@@ -182,8 +182,8 @@ define( require => {
     }
 
     /**
-     * Emits a single event.
-     * This method is called many times in a simulation and must be well-optimized.
+     * Emits a single event.  This method is called many times in a simulation and must be well-optimized.  Listeners
+     * are notified in the order they were added via addListener.
      * @params - expected parameters are based on options.argumentTypes, see constructor
      * @public
      */

js/Property.js

@@ -309,7 +309,8 @@ define( require => {
 
     /**
      * Adds listener and calls it immediately. If listener is already registered, this is a no-op. The initial
-     * notification provides the current value for newValue and null for oldValue.
+     * notification provides the current value for newValue and null for oldValue.  On value changes, listeners
+     * are called back in the order they were added.
      *
      * @param {function} listener a function of the form listener(newValue,oldValue)
      * @public
@@ -320,10 +321,9 @@ define( require => {
     }
 
     /**
-     * Add an listener to the Property, without calling it back right away.
-     * This is used when you need to register a listener without an immediate callback.
-     *
-     * @param {function} listener - a function with a single argument, which is the value of the property at the time the function is called.
+     * Add an listener to the Property, without calling it back right away. This is used when you need to register a
+     * listener without an immediate callback. On value changes, listeners are called back in the order they were added.
+     * @param {function} listener - a function with a single argument, which is the current value of the Property.
      * @public
      */
     lazyLink( listener ) {