added explicit comment about file transport to mail config

fixes #3694

config/web.php

@@ -19,6 +19,9 @@ $config = [
         ],
         'mail' => [
             'class' => 'yii\swiftmailer\Mailer',
+            // send all mails to a file by default. You have to set
+            // 'useFileTransport' to false and configure a transport
+            // for the mailer to send real emails.
             'useFileTransport' => true,
         ],
         'log' => [