update docs for hook registration methods

Author: yyx990803

docs/en/api/router-instance.md

@@ -28,6 +28,7 @@
 
   Add global navigation guards. See [Navigation Guards](../advanced/navigation-guards.md).
 
+  In 2.5.0+ all three methods return a function that removes the registered guard/hook.
 
 - **router.push(location, onComplete?, onAbort?)**
 - **router.replace(location, onComplete?, onAbort?)**