Clarify allowed_sender/allowed_recipient empty string behavior

smtprelay.ini

@@ -35,10 +35,12 @@
 ;allowed_nets = 127.0.0.0/8 ::1/128
 
 ; Regular expression for valid FROM EMail addresses
+; If set to "", then any sender is permitted.
 ; Example: ^(.*)@localhost.localdomain$
 ;allowed_sender =
 
 ; Regular expression for valid TO EMail addresses
+; If set to "", then any recipient is permitted.
 ; Example: ^(.*)@localhost.localdomain$
 ;allowed_recipients =