tick/sched: Add function description for tick_nohz_next_event()

The return value of tick_nohz_next_event() is not obvious at the first
glance. Add a kernel-doc compatible function description which also covers
return values.

Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240123164702.55612-4-anna-maria@linutronix.de