Update rigctl.1 man page to make -L a bit clearer

doc/man1/rigctl.1

@@ -183,7 +183,7 @@ below.
 .BR \-L ", " \-\-show\-conf
 List all config parameters for the radio defined with
 .B \-m
-above.
+above. Note the dummy device has no serial parameters.
 .
 .TP
 .BR \-C ", " \-\-set\-conf = \fIparm=val\fP [ \fI,parm=val\fP ]