ABI: sysfs-devices-system-cpu: point to the right docs

The cpuidle doc was split on two, one at the admin guide
and another one at the driver API guide. Instead of pointing
to a non-existent file, point to both (admin guide being
the first one).

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>