choom: improve docs

It's not obvious from the current docs that you have to explicitly
split command line options and wanted commands.

Addresses: https://github.com/karelzak/util-linux/issues/833
Signed-off-by: Karel Zak <kzak@redhat.com>

sys-utils/choom.1

@@ -15,6 +15,7 @@ choom \- display and adjust OOM-killer score.
 .B choom
 .B \-n
 .I number
+.B [\-\-]
 .IR command\  [ argument ...]
 
 .SH DESCRIPTION

sys-utils/choom.c

@@ -37,7 +37,7 @@ static void __attribute__((__noreturn__)) usage(void)
 	fprintf(out,
 	      _(" %1$s [options] -p pid\n"
 		" %1$s [options] -n number -p pid\n"
-		" %1$s [options] -n number command [args...]]\n"),
+		" %1$s [options] -n number [--] command [args...]]\n"),
 		program_invocation_short_name);
 
 	fputs(USAGE_SEPARATOR, out);