docs($rootScope): document scope properties

src/ng/rootScope.js

@@ -139,10 +139,26 @@ function $RootScopeProvider(){
     /**
      * @ngdoc property
      * @name $rootScope.Scope#$id
-     * @returns {number} Unique scope ID (monotonically increasing alphanumeric sequence) useful for
-     *   debugging.
+     *
+     * @description
+     * Unique scope ID (monotonically increasing) useful for debugging.
      */
 
+     /**
+      * @ngdoc property
+      * @name $rootScope.Scope#$parent
+      *
+      * @description
+      * Reference to the parent scope.
+      */
+
+      /**
+       * @ngdoc property
+       * @name $rootScope.Scope#$root
+       *
+       * @description
+       * Reference to the root scope.
+       */
 
     Scope.prototype = {
       constructor: Scope,