genirq: Add kerneldoc for irq_disable.

Document the lazy disable functionality. comment based on changelog of
d209a699

Signed-off-by: Andreas Fenkart <andreas.fenkart@streamunlimited.com>
Cc: balbi@ti.com
Link: http://lkml.kernel.org/r/1368181290-1583-1-git-send-email-andreas.fenkart@streamunlimited.com


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>