HID: logitech-hidpp: document sysfs interface

Descriptions have been collected from git commit logs.
Signed-off-by: Aishwarya Pant <aishpant@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

Documentation/ABI/testing/sysfs-driver-hid-logitech-hidpp

+What:		/sys/bus/hid/drivers/logitech-hidpp-device/<dev>/range
+Date:		Jan, 2016
+KernelVersion:	4.6
+Contact:	linux-input@vger.kernel.org
+Description:
+		(RW) This attribute controls the amount of 'turn' permitted in
+		Logitech G920 wheel. Reading from the file shows the current
+		range of the steering wheel. Writing a value within the min and
+		max boundary sets the range of the wheel.
+
+What:		/sys/bus/hid/drivers/logitech-hidpp-device/<dev>/builtin_power_supply
+Date:		Apr, 2017
+KernelVersion:	4.12
+Contact:	linux-input@vger.kernel.org
+Description:
+		Presence of this file indicates that HID++ driver is capable of
+		handling battery properties in the kernel. This way, upower can
+		add a udev rule to decide whether or not it should use the
+		internal unifying support or the generic kernel one.