perf tools: Update ioctl documentation for PERF_IOC_FLAG_GROUP

The ioctl interface of perf event fd receives 3 arguments to control
event group behavior but it lacked documentation.
Signed-off-by: Namhyung Kim <namhyung.kim@lge.com>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1338443506-25009-2-git-send-email-namhyung.kim@lge.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/design.txt

@@ -409,14 +409,15 @@ Counters can be enabled and disabled in two ways: via ioctl and via
 prctl.  When a counter is disabled, it doesn't count or generate
 events but does continue to exist and maintain its count value.
 
-An individual counter or counter group can be enabled with
+An individual counter can be enabled with
 
-	ioctl(fd, PERF_EVENT_IOC_ENABLE);
+	ioctl(fd, PERF_EVENT_IOC_ENABLE, 0);
 
 or disabled with
 
-	ioctl(fd, PERF_EVENT_IOC_DISABLE);
+	ioctl(fd, PERF_EVENT_IOC_DISABLE, 0);
 
+For a counter group, pass PERF_IOC_FLAG_GROUP as the third argument.
 Enabling or disabling the leader of a group enables or disables the
 whole group; that is, while the group leader is disabled, none of the
 counters in the group will count.  Enabling or disabling a member of a