doc(routing): event.allowLeave() is on the PreLeave event since 0.4.21

Closes #1469

Author: vicb

lib/routing/module.dart

@@ -80,29 +80,29 @@
  *        EditRecipe(RouteProvider routeProvider) {
  *          RouteHandle route = routeProvider.route.newHandle();
  *          _loadRecipe(route);
- *          route.onLeave.listen((RouteEvent event) {
+ *          route.onPreLeave.listen((RouteEvent event) {
  *            event.allowLeave(_checkIfOkToLeave());
  *          });
  *        }
  *
  *        /// Check if the editor has unsaved contents and if necessary ask
  *        /// the user if OK to leave this page.
- *        Future<bool> _checkIfOkToLeave() {/* ... */}
+ *        Future<bool> _checkIfOkToLeave() => /* ... */;
  *
  *        detach() {
  *          route.discard();
  *        }
  *      }
  *
- *  [Route.onLeave] event is triggered when the browser is routed from an
+ *  [Route.onPreLeave] event is triggered when the browser is routed from an
  *  active route to a different route. The active route can delay and
  *  potentially veto the navigation by passing a [Future<bool>] to
- *  [RouteEvent.allowLeave].
+ *  [RoutePreLeaveEvent.allowLeave].
  *
  *  Notice that we create a [RouteHandle] for our route. [RouteHandle] are
  *  a convenient wrapper around [Route] that makes unsubscribing route events
  *  easier. For example, notice that we didn't need to manually call
- *  [StreamSubscription.cancel] for subscription to [Route.onLeave]. Calling
+ *  [StreamSubscription.cancel] for subscription to [Route.onPreLeave]. Calling
  *  [RouteHandle.discard] unsubscribes all listeners created for the handle.
  *
  * ## Hierarchical Routes

lib/routing/ng_view.dart

@@ -177,7 +177,7 @@ class NgView implements DetachAware, RouteProvider {
  *         route.onEnter.listen((RouteEvent e) {
  *           // Do something when the route is activated.
  *         });
- *         route.onLeave.listen((RouteEvent e) {
+ *         route.onPreLeave.listen((RouteEvent e) {
  *           // Do something when the route is de-activated.
  *           e.allowLeave(allDataSaved());
  *         });
@@ -193,7 +193,7 @@ class NgView implements DetachAware, RouteProvider {
  *       }
  *     }
  *
- * If user component is used outside of ng-view directive then
+ * If user component is used outside of `ng-view` directive then
  * injected [RouteProvider] will be null.
  */
 abstract class RouteProvider {