Improved help text of piped frontend demo

test/frontend_pipes_xorriso.c

@@ -46,6 +46,9 @@ static int usage()
 "Meta command",
 "  @drain_sieve",
 "reports all recorded results of all installed message sieve filter rules.",
+"For illustration perform this xorriso command sequence",
+"  -msg_op start_sieve - -outdev /dev/sr0 -msg_op clear_sieve - -toc",
+"and then @drain_sieve.", 
 "@END@"
     };
     int i;