Documentation: ABI: /sys/devices/system/cpu/cpuidle/

Document cpuidle sysfs attributes by reading code, Documentation/cpuidle/,
and git logs.

Cc: Venki Pallipadi <venkatesh.pallipadi@intel.com>
Cc: Len Brown <lenb@kernel.org>
Signed-off-by: Alex Chiang <achiang@hp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>