rtc: tweak driver documentation for rtc periodic

The max_user_freq member is not really meant for RTC drivers to modify, so
update the rtc documentation so drivers writers know what is expected of
them when handling periodic events.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Acked-by: Alessandro Zummo <a.zummo@towertech.it>
Cc: David Brownell <david-b@pacbell.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/rtc.txt

@@ -180,9 +180,10 @@ driver returns ENOIOCTLCMD.  Some common examples:
     *	RTC_IRQP_SET, RTC_IRQP_READ: the irq_set_freq function will be called
 	to set the frequency while the framework will handle the read for you
 	since the frequency is stored in the irq_freq member of the rtc_device
-	structure.  Also make sure you set the max_user_freq member in your
-	initialization routines so the framework can sanity check the user
-	input for you.
+	structure.  Your driver needs to initialize the irq_freq member during
+	init.  Make sure you check the requested frequency is in range of your
+	hardware in the irq_set_freq function.  If you cannot actually change
+	the frequency, just return -ENOTTY.
 
 If all else fails, check out the rtc-test.c driver!