Add notice for local_forcetls because it cannot be used when unencrypted

SMTP port is used

smtp-proxy.ini

@@ -18,7 +18,8 @@
 ;local_key  = smtpd.key
 
 ; Enforce encrypted connection on STARTTLS ports before
-; accepting mails from client.
+; accepting mails from client. Cannot be used with plain
+; non encrypted listeners.
 ;local_forcetls = false
 
 ; Relay all mails to this SMTP server