docs: add note for filter $aliases

Author: kenjis

docs/quick_start_guide/using_session_auth.md

@@ -106,5 +106,12 @@ public $globals = [
 ];
 ```
 
+!!! note
+
+    The filter `$aliases` that Shield provides are automatically added for you by the
+    [Registrar](https://codeigniter.com/user_guide/general/configuration.html#registrars)
+    class located at **src/Config/Registrar.php**. So you don't need to add in
+    your **app/Config/Filters.php**.
+
 Check your filters with the [spark routes](https://codeigniter.com/user_guide/incoming/routing.html#spark-routes)
 command.