Documentation: admin-guide: pm: Document intel_idle C1 demotion

Document the intel_idle driver sysfs file for enabling/disabling C1
demotion.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Link: https://patch.msgid.link/20250317135541.1471754-3-dedekind1@gmail.com


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