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>

doc/reference/kernel/other/cpu_idle.rst

@@ -25,8 +25,9 @@ Making the CPU idle
 ===================
 
 Making the CPU idle is simple: call the k_cpu_idle() API. The CPU will stop
-executing instructions until an event occurs. Make sure interrupts are not
-locked before invoking it. Most likely, it will be called within a loop.
+executing instructions until an event occurs. Most likely, the function will
+be called within a loop. Note that in certain architectures, upon return,
+k_cpu_idle() unconditionally unmasks interrupts.
 
 .. code-block:: c