Improve documentation for animationFrame.

tools/dom/templates/html/impl/impl_Window.darttemplate

Index: tools/dom/templates/html/impl/impl_Window.darttemplate
diff --git a/tools/dom/templates/html/impl/impl_Window.darttemplate b/tools/dom/templates/html/impl/impl_Window.darttemplate
index 62779ca7b449e07443a0716e6fa69bc4590748a1..9fead2690e7c941e7274a60b21f283a0a14c3261 100644
--- a/tools/dom/templates/html/impl/impl_Window.darttemplate
+++ b/tools/dom/templates/html/impl/impl_Window.darttemplate
@@ -48,12 +48,17 @@ $endif
   }
 
   /**
-   * 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.
    */