ACPI: dock: document sysfs interface

Description has been collected from git commit history and reading
through code.
Signed-off-by: Aishwarya Pant <aishpant@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Documentation/ABI/testing/sysfs-devices-platform-dock

+What:		/sys/devices/platform/dock.N/docked
+Date:		Dec, 2006
+KernelVersion:	2.6.19
+Contact:	linux-acpi@vger.kernel.org
+Description:
+		(RO) Value 1 or 0 indicates whether the software believes the
+		laptop is docked in a docking station.
+
+What:		/sys/devices/platform/dock.N/undock
+Date:		Dec, 2006
+KernelVersion:	2.6.19
+Contact:	linux-acpi@vger.kernel.org
+Description:
+		(WO) Writing to this file causes the software to initiate an
+		undock request to the firmware.
+
+What:		/sys/devices/platform/dock.N/uid
+Date:		Feb, 2007
+KernelVersion:	v2.6.21
+Contact:	linux-acpi@vger.kernel.org
+Description:
+		(RO) Displays the docking station the laptop is docked to.
+
+What:		/sys/devices/platform/dock.N/flags
+Date:		May, 2007
+KernelVersion:	v2.6.21
+Contact:	linux-acpi@vger.kernel.org
+Description:
+		(RO) Show dock station flags, useful for checking if undock
+		request has been made by the user (from the immediate_undock
+		option).
+
+What:		/sys/devices/platform/dock.N/type
+Date:		Aug, 2008
+KernelVersion:	v2.6.27
+Contact:	linux-acpi@vger.kernel.org
+Description:
+		(RO) Display the dock station type- dock_station, ata_bay or
+		battery_bay.