staging/android: Add kerneldoc to one function in alarm-dev.c

Sorry. I thought that this would be a nice easy class to document fully.
Turns out I was very wrong - I will have to research the Linux alarm and
timer subsystem one day next week and try again.

Here is what I started out with, anyway. It's not much, but it's better
than nothing!

Signed-off-by: Cruz Julian Bishop <cruzjbishop@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>