kernel: cpu idle: add info about IRQ unlocking in k_cpu_idle

We add a note in k_cpu_idle() documentation, stressing that for
certain architectures. the function unmasks interrupts
unconditionally before returning. In the
documentation of the architecture-specific API (arch_cpu_idle)
we describe the expected behavior with regards to the wake-up
event.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>