docs: add comment

Author: kenjis

user_guide_src/source/database/transactions/003.php

@@ -1,5 +1,7 @@
 <?php
 
+// DBDebug in the Database Config must be false.
+
 $this->db->transStart();
 $this->db->query('AN SQL QUERY...');
 $this->db->query('ANOTHER QUERY...');