doc: reference: kernel: remove inconsistent comment for k_cpu_idle

k_cpu_idle() API does not need to be invoked with interrupts
unlocked; it is actually invoked with interrupts locked by
the kernel CPU idling mechanism. In most architectures, the
function is, actually, un-locking interrupts itself. We need
to remove this comment from the documentation of the CPU
idle API. We add a note about the un-locking of interrupts.

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