Update redirect-and-alias.md (#1588)

* Update redirect-and-alias.md

Added note about navigation guards not being applied on redirects. 

Background: I just tried doing a conditional redirect where I rely on user data being loaded into the vuex store by the route guards. But since they are only applied after the redirect (on the actual route that you are being redirected to), the data was never there.

* Update redirect-and-alias.md

Author: Christof Leuenberger

Diff for: docs/en/essentials/redirect-and-alias.md

@@ -35,7 +35,9 @@ const router = new VueRouter({
 })
 ```
 
-For other advanced usage, check out the [example](https://github.com/vuejs/vue-router/blob/dev/examples/redirect/app.js).
+Note that [Navigation Guards](../advanced/navigation-guards.md) are not applied on the route that redirects, only on its target. In the example below, adding a `beforeEnter` or `beforeLeave` guard to the `/a` route would not have any effect.
+
+For other advanced usage, checkout the [example](https://github.com/vuejs/vue-router/blob/dev/examples/redirect/app.js).
 
 ### Alias