Added some more documentation to top level Window.

tools/dom/templates/html/dartium/html_dartium.darttemplate

Index: tools/dom/templates/html/dartium/html_dartium.darttemplate
diff --git a/tools/dom/templates/html/dartium/html_dartium.darttemplate b/tools/dom/templates/html/dartium/html_dartium.darttemplate
index 4116036bbf3cb1c332ee0a6e4827ca9a25067487..04480437afd6b58300432b77ef2ce2df82d71bd0 100644
--- a/tools/dom/templates/html/dartium/html_dartium.darttemplate
+++ b/tools/dom/templates/html/dartium/html_dartium.darttemplate
@@ -59,6 +59,19 @@ part '$AUXILIARY_DIR/native_DOMImplementation.dart';
 
 Window _window;
 
+/**
+ * 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.
+ *
+ * If the web page has any `<iframe>` elements, then each `iframe` has its own
+ * [Window] object, which is accessible only to that `iframe`.
+ *
+ * See also:
+ *
+ *   * [Window](https://developer.mozilla.org/en-US/docs/Web/API/window) from MDN.
+ */
 Window get window {
   if (_window != null) {
     return _window;
@@ -69,6 +82,9 @@ Window get window {
 
 HtmlDocument _document;
 
+/**
+ * Root node for all content in a web page.
+ */
 HtmlDocument get document {
   if (_document != null) {
     return _document;