docs: better routeBeforeUpdate

Close #2951

Author: posva

Diff for: docs/guide/advanced/navigation-guards.md

@@ -106,11 +106,11 @@ const Foo = {
     // because it has not been created yet when this guard is called!
   },
   beforeRouteUpdate (to, from, next) {
-    // called when the route that renders this component has changed,
-    // but this component is reused in the new route.
+    // called when the route that renders this component has changed.
+    // This component being reused (by using an explicit `key`) in the new route or not doesn't change anything.
     // For example, for a route with dynamic params `/foo/:id`, when we
     // navigate between `/foo/1` and `/foo/2`, the same `Foo` component instance
-    // will be reused, and this hook will be called when that happens.
+    // will be reused (unless you provided a `key` to `<router-view>`), and this hook will be called when that happens.
     // has access to `this` component instance.
   },
   beforeRouteLeave (to, from, next) {