alarmtimer: Update kerneldoc

Update kerneldoc comments to reflect the actual arguments and return values
of the documented functions.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20210202013457.3482388-1-alexandre.belloni@bootlin.com