perf_counter tools: Document '--' option parsing terminator

Signed-off-by: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
diff --git a/Documentation/perf_counter/builtin-record.c b/Documentation/perf_counter/builtin-record.c
index 4a06866..23d1224 100644
--- a/Documentation/perf_counter/builtin-record.c
+++ b/Documentation/perf_counter/builtin-record.c
@@ -397,7 +397,8 @@
 }
 
 static const char * const record_usage[] = {
-	"perf record [<options>] <command>",
+	"perf record [<options>] [<command>]",
+	"perf record [<options>] -- <command> [<options>]",
 	NULL
 };