docs(rootScope): improve grammar and clarity

Author: petebacondarwin

src/ng/rootScope.js

@@ -55,8 +55,10 @@
  * @description
  *
  * Every application has a single root {@link ng.$rootScope.Scope scope}.
- * All other scopes are child scopes of the root scope. Scopes provide mechanism for watching the model and provide
- * event processing life-cycle. See {@link guide/scope developer guide on scopes}.
+ * All other scopes are descendant scopes of the root scope. Scopes provide separation
+ * between the model and the view, via a mechanism for watching the model for changes.
+ * They also provide an event emission/broadcast and subscription facility. See the
+ * {@link guide/scope developer guide on scopes}.
  */
 function $RootScopeProvider(){
   var TTL = 10;