Added some more documentation to top level Window.

tools/dom/templates/html/dart2js/html_dart2js.darttemplate

Index: tools/dom/templates/html/dart2js/html_dart2js.darttemplate
diff --git a/tools/dom/templates/html/dart2js/html_dart2js.darttemplate b/tools/dom/templates/html/dart2js/html_dart2js.darttemplate
index 89cc40e59937a17ea35fe312e16d268c56e4c874..00ad16416575a56de71395f50f9e65f0f658751a 100644
--- a/tools/dom/templates/html/dart2js/html_dart2js.darttemplate
+++ b/tools/dom/templates/html/dart2js/html_dart2js.darttemplate
@@ -63,12 +63,22 @@ part '$AUXILIARY_DIR/_ListIterators.dart';
 
 
 /**
- * The top-level Window object.
+ * Top-level container for a web page, which is usually a browser tab or window.
+ *
+ * Each web page loaded in the browser has its own [Window], which is a
+ * container for the web page.
+ *
+ * This getter usually retuns the only [Window] on the page, but any `<iframe>`
+ * elements have their own [Window].
+ *
+ * See also:
+ *
+ *   * [Window](https://developer.mozilla.org/en-US/docs/Web/API/window) from MDN.
  */
 Window get window => JS('Window', 'window');
 
 /**
- * The top-level Document object.
+ * Root node for all content in a web page.
  */
 HtmlDocument get document => JS('HtmlDocument', 'document');