PM: Document device power attributes in sysfs

There are sysfs attributes in /sys/devices/.../power/ that haven't
been documented yet in Documentation/ABI/.  Document them as
appropriate.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>

diff --git a/Documentation/ABI/testing/sysfs-devices-power b/Documentation/ABI/testing/sysfs-devices-power
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+What:          /sys/devices/.../power/
+Date:          January 2009
+Contact:       Rafael J. Wysocki <rjw@sisk.pl>
+Description:
+               The /sys/devices/.../power directory contains attributes
+               allowing the user space to check and modify some power
+               management related properties of given device.
+
+What:          /sys/devices/.../power/wakeup
+Date:          January 2009
+Contact:       Rafael J. Wysocki <rjw@sisk.pl>
+Description:
+               The /sys/devices/.../power/wakeup attribute allows the user
+               space to check if the device is enabled to wake up the system
+               from sleep states, such as the memory sleep state (suspend to
+               RAM) and hibernation (suspend to disk), and to enable or disable
+               it to do that as desired.
+
+               Some devices support "wakeup" events, which are hardware signals
+               used to activate the system from a sleep state.  Such devices
+               have one of the following two values for the sysfs power/wakeup
+               file:
+
+               + "enabled\n" to issue the events;
+               + "disabled\n" not to do so;
+
+               In that cases the user space can change the setting represented
+               by the contents of this file by writing either "enabled", or
+               "disabled" to it.
+
+               For the devices that are not capable of generating system wakeup
+               events this file contains "\n".  In that cases the user space
+               cannot modify the contents of this file and the device cannot be
+               enabled to wake up the system.
+
+What:          /sys/devices/.../power/control
+Date:          January 2009
+Contact:       Rafael J. Wysocki <rjw@sisk.pl>
+Description:
+               The /sys/devices/.../power/control attribute allows the user
+               space to control the run-time power management of the device.
+
+               All devices have one of the following two values for the
+               power/control file:
+
+               + "auto\n" to allow the device to be power managed at run time;
+               + "on\n" to prevent the device from being power managed;
+
+               The default for all devices is "auto", which means that they may
+               be subject to automatic power management, depending on their
+               drivers.  Changing this attribute to "on" prevents the driver
+               from power managing the device at run time.  Doing that while
+               the device is suspended causes it to be woken up.