Document the document class.

sdk/lib/html/templates/html/impl/impl_Document.darttemplate

Index: sdk/lib/html/templates/html/impl/impl_Document.darttemplate
diff --git a/sdk/lib/html/templates/html/impl/impl_Document.darttemplate b/sdk/lib/html/templates/html/impl/impl_Document.darttemplate
index 0ddb96545a01bfdd604e66aba7f4316746f8d429..3971f831a21c4ebb5f8e019a9cd0e616381450e6 100644
--- a/sdk/lib/html/templates/html/impl/impl_Document.darttemplate
+++ b/sdk/lib/html/templates/html/impl/impl_Document.darttemplate
@@ -5,12 +5,35 @@
 part of html;
 
 /// @domName $DOMNAME
+/**
+ * The base class for all documents.
+ *
+ * Each web page loaded in the browser has its own [Document] object, which is
+ * typically an [HtmlDocument].
+ *
+ * If you aren't comfortable with DOM concepts, see the Dart tutorial
+ * [Target 2: Connect Dart & HTML](http://www.dartlang.org/docs/tutorials/connect-dart-html/).
+ */
 class $CLASSNAME extends Node $NATIVESPEC
 {
 
 $!MEMBERS
-  // TODO(jacobr): implement all Element methods not on Document.
 
+  /**
+   * Finds the first descendant element of this document that matches the
+   * specified group of selectors.
+   *
+   * Unless your webpage contains multiple documents, the top-level query
+   * method behaves the same as this method, so you should use it instead to
+   * save typing a few characters.
+   *
+   * [selectors] should be a string using CSS selector syntax.
+   *     var element1 = document.query('.className');
+   *     var element2 = document.query('#id');
+   *
+   * For details about CSS selector syntax, see the
+   * [CSS selector specification](http://www.w3.org/TR/css3-selectors/).
+   */
   Element query(String selectors) {
     // It is fine for our RegExp to detect element id query selectors to have
     // false negatives but not false positives.
@@ -20,6 +43,20 @@ $!MEMBERS
     return $dom_querySelector(selectors);
   }
 
+  /**
+   * Finds all descendant elements of this document that match the specified
+   * group of selectors.
+   *
+   * Unless your webpage contains multiple documents, the top-level queryAll
+   * method behaves the same as this method, so you should use it instead to
+   * save typing a few characters.
+   *
+   * [selectors] should be a string using CSS selector syntax.
+   *     var items = document.queryAll('.itemClassName');
+   *
+   * For details about CSS selector syntax, see the
+   * [CSS selector specification](http://www.w3.org/TR/css3-selectors/).
+   */
   List<Element> queryAll(String selectors) {
     if (new RegExp("""^\\[name=["'][^'"]+['"]\\]\$""").hasMatch(selectors)) {
       final mutableMatches = $dom_getElementsByName(