rtc: update documentation wrt RTC_PIE/irq_set_state

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Cc: David Brownell <david-b@pacbell.net>
Signed-off-by: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/Documentation/rtc.txt b/Documentation/rtc.txt
index 8deffcd..102ce96 100644
--- a/Documentation/rtc.txt
+++ b/Documentation/rtc.txt
@@ -185,6 +185,8 @@
 	hardware in the irq_set_freq function.  If it isn't, return -EINVAL.  If
 	you cannot actually change the frequency, do not define irq_set_freq.
 
+    *	RTC_PIE_ON, RTC_PIE_OFF: the irq_set_state function will be called.
+
 If all else fails, check out the rtc-test.c driver!