rtc: tweak driver documentation for rtc periodic

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>