Improve documentation for animationFrame.

sdk/lib/html/dart2js/html_dart2js.dart

Index: sdk/lib/html/dart2js/html_dart2js.dart
diff --git a/sdk/lib/html/dart2js/html_dart2js.dart b/sdk/lib/html/dart2js/html_dart2js.dart
index 9e18c6d783f0a2414c9a70292b2cf5d53ecc6565..ef3ef846ca22e4e4b61d3ae0a5a2cbb8fcf70ee9 100644
--- a/sdk/lib/html/dart2js/html_dart2js.dart
+++ b/sdk/lib/html/dart2js/html_dart2js.dart
@@ -22922,12 +22922,17 @@ class Window extends EventTarget implements WindowBase native "Window,DOMWindow"
   }
 
   /**
-   * Returns a Future that completes just before the window is about to repaint
-   * so the user can draw an animation frame
+   * Returns a Future that completes just before the window is about to 
+   * repaint so the user can draw an animation frame.
    *
    * If you need to later cancel this animation, use [requestAnimationFrame]
    * instead.
    *
+   * The [Future] completes to a timestamp that represents a floating
+   * point value of the number of milliseconds that have elapsed since the page 
+   * started to load (which is also the timestamp at this call to 
+   * animationFrame).
+   *
    * Note: The code that runs when the future completes should call
    * [animationFrame] again for the animation to continue.
    */