Add documentation for two attributes, status and hrv, in
Documentation/ABI/testing/sysfs-bus-acpi.

Compiled from git logs and the ACPI specification.
Signed-off-by: NAishwarya Pant <aishpant@gmail.com>
[ rjw: Minor changes ]
Signed-off-by: NRafael J. Wysocki <rafael.j.wysocki@intel.com>