Documentation: ABI: testing: document the new cpuidle sysfs file

Mention the new 'intel_c1_demotion' sysfs file in the "cpuidle" section
and refer to "Documentation/admin-guide/pm/intel_idle.rst" for more
information.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Link: https://patch.msgid.link/20250317135541.1471754-4-dedekind1@gmail.com
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Author: dedekind

Documentation/ABI/testing/sysfs-devices-system-cpu

@@ -111,6 +111,7 @@ What:		/sys/devices/system/cpu/cpuidle/available_governors
 		/sys/devices/system/cpu/cpuidle/current_driver
 		/sys/devices/system/cpu/cpuidle/current_governor
 		/sys/devices/system/cpu/cpuidle/current_governer_ro
+		/sys/devices/system/cpu/cpuidle/intel_c1_demotion
 Date:		September 2007
 Contact:	Linux kernel mailing list <linux-kernel@vger.kernel.org>
 Description:	Discover cpuidle policy and mechanism
@@ -132,7 +133,11 @@ Description:	Discover cpuidle policy and mechanism
 
 		current_governor_ro: (RO) displays current idle policy.
 
-		See Documentation/admin-guide/pm/cpuidle.rst and
+		intel_c1_demotion: (RW) enables/disables the C1 demotion
+		feature on Intel CPUs.
+
+		See Documentation/admin-guide/pm/cpuidle.rst,
+		Documentation/admin-guide/pm/intel_idle.rst, and
 		Documentation/driver-api/pm/cpuidle.rst for more information.