intel_idle: Clean up definitions of cpuidle callbacks

Add proper kerneldoc descriptions to intel_idle() and
intel_idle_s2idle(), annotate the latter with __cpuidle and
reorder the declarations of local variables in both of them to
reflect the mwait_idle_with_hints() arguments order.

No intentional functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>