docs: improve in-component guard examples (#1704)

* Improve in-component guard examples

* Update navigation-guards.md

Author: LinusBorg

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

@@ -116,7 +116,28 @@ beforeRouteEnter (to, from, next) {
 }
 ```
 
-You can directly access `this` inside `beforeRouteLeave`. The leave guard is usually used to prevent the user from accidentally leaving the route with unsaved edits. The navigation can be canceled by calling `next(false)`.
+Note that `beforeRouteEnter` is the only hook that supports passing a callback to `next`. For `beforeRouteUpdate` and `beforeRouteLeave`, `this` is already available, so passing a callback is unnecessary and therefore *not supported*:
+
+```js
+beforeRouteUpdate (to, from, next) {
+  // just use `this`
+  this.name = to.params.name
+  next()
+}
+```
+
+The **leave guard** is usually used to prevent the user from accidentally leaving the route with unsaved edits. The navigation can be canceled by calling `next(false)`.
+
+```js
+beforeRouteLeave (to, from , next) {
+  const answer = window.confirm('Do you really want to leave? you have unsaved changes!')
+  if (answer) {
+    next()
+  } else {
+    next(false)
+  }
+}
+```
 
 ### The Full Navigation Resolution Flow