Add some doco to app window onClosed event explaining how to use it.

chrome/common/extensions/api/app_window.idl

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Use the <code>chrome.app.window</code> API to create windows. Windows
 // have an optional frame with title bar and size controls. They are not
 // associated with any Chrome browser windows.
 namespace app.window {
 
   // Previously named Bounds.
@@ skipping 393 matching lines @@
 
     // Gets an $(ref:AppWindow) with the given id. If no window with the given id
     // exists null is returned. This method is new in Chrome 33.
     [nocompile] static AppWindow get(DOMString id);
   };
 
   interface Events {
     // Fired when the window is resized.
     [nocompile] static void onBoundsChanged();
 
-    // Fired when the window is closed.
+    // Fired when the window is closed. Note, this should be listened to from
+    // a window other than the window being closed, for example from the
+    // background page. This is because the window being closed will be in the
+    // process of being torn down when the event is fired, which means not all
+    // APIs in the window's script context will be functional.
     [nocompile] static void onClosed();
 
     // Fired when the window is fullscreened.
     [nocompile] static void onFullscreened();
 
     // Fired when the window is maximized.
     [nocompile] static void onMaximized();
 
     // Fired when the window is minimized.
     [nocompile] static void onMinimized();
 
     // Fired when the window is restored from being minimized or maximized.
     [nocompile] static void onRestored();
   };
 };