docs: add comment

Author: kenjis

user_guide_src/source/installation/upgrade_sessions/001.php

@@ -2,7 +2,7 @@
 
 $session = session();
 
-$_SESSION['item'];
+$_SESSION['item'];  // But we do not recommend to use superglobal directly.
 $session->get('item');
 $session->item;
 session('item');