Adjust the -M help so it specifies the range of accepted values (#67).

src/main/help.c

@@ -372,7 +372,7 @@ void display_help(void)
 		 N_("show progress of process PID"),
 		 { 0, 0, 0, 0} },
 #endif				/* PV_REMOTE_CONTROL */
-		{ "-M", "--monitor", N_("SIDE"),
+		{ "-M", "--monitor", "in|0|out|1|both|2",
 		 N_("run a command and monitor its standard input, output, or both"),
 		 { 0, 0, 0, 0} },
 		{ "", NULL, NULL, NULL, { 0, 0, 0, 0} },