[docs] improve docs on how hooks work (close #259)

Author: yyx990803

docs/en/api/before-each.md

@@ -4,6 +4,8 @@ Add a global before hook to the router, which will be called before every route
 
 You can add multiple global before hooks to the same router. These hooks will be called in the order of creation. Since hooks can be resolved asynchronously, the next hook won't be called until the previous one resolves.
 
+Global before hooks are resolved following the same [Hook resolution rules](../pipeline/hooks.html#hook-resolution-rules).
+
 ### Arguments
 
 - `hook {Function}`

docs/en/route.md

@@ -45,12 +45,16 @@ When `/a` is matched, `$route.auth` will be `true`. This allows us to perform au
 
 ``` js
 router.beforeEach(function (transition) {
-  if (transition.to.auth) {
-    // do authentication...
+  if (transition.to.auth && !authenticated) {
+    transition.redirect('/login')
+  } else {
+    transition.next()
   }
 })
 ```
 
+> See [API](api/before-each.md) for how the `beforeEach` hook works.
+
 When a nested route is matched, all custom fields will be merged on to the same `$route` object. When a sub route and a parent route has the same custom field, the sub route's value will overwrite the parent's.
 
 ### Using in Templates