rtc: update documentation wrt RTC_PIE/irq_set_state

Signed-off-by: NMike Frysinger <vapier@gentoo.org>
Cc: David Brownell <david-b@pacbell.net>
Signed-off-by: NAlessandro Zummo <a.zummo@towertech.it>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

Documentation/rtc.txt

@@ -185,6 +185,8 @@ driver returns ENOIOCTLCMD.  Some common examples:
 	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!