Code review fix: focusedEditorChange was passing wrong last arg to its

listeners.
And add a couple more docs.

src/editor/Editor.js

@@ -631,6 +631,7 @@ define(function (require, exports, module) {
 
         this._codeMirror.setOption("onBlur", function () {
             self._focused = false;
+            // EditorManager only cares about other Editors gaining focus, so we don't notify it of anything here
         });
     };
 

src/editor/EditorManager.js

@@ -380,16 +380,18 @@ define(function (require, exports, module) {
 
     /**
      * @private
+     * @param {?Editor} current
      */
     function _notifyFocusedEditorChanged(current) {
         // Skip if the Editor that gained focus was already the most recently focused editor.
         // This may happen e.g. if the window loses then regains focus.
         if (_lastFocusedEditor === current) {
             return;
         }
+        var previous = _lastFocusedEditor;
         _lastFocusedEditor = current;
 
-        $(exports).triggerHandler("focusedEditorChange", [current, _lastFocusedEditor]);
+        $(exports).triggerHandler("focusedEditorChange", [current, previous]);
     }
 
     /**