Adding a comment and renaming on_bulk_change to on_change

IPython/html/static/widgets/js/widget.js

@@ -266,7 +266,12 b' define(["widgets/js/manager",'
             }
         },
 
-        on_bulk_change: function(keys, callback, context) {
+        on_change: function(keys, callback, context) {
+            // on_change(["key1", "key2"], foo, context) differs from
+            // on("change:key1 change:key2", foo, context).
+            // If the widget attributes key1 and key2 are both modified, 
+            // the second form will result in foo being called twice
+            // while the first will call foo only once.
             this.on('change', function() {
                 if (keys.some(this.hasChanged, this)) {
                     callback.apply(context);