Input: document INPUT_PROP_TOPBUTTONPAD

Signed-off-by: NPeter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: NDmitry Torokhov <dmitry.torokhov@gmail.com>

Documentation/input/event-codes.txt

@@ -281,6 +281,19 @@ gestures can normally be extracted from it.
 If INPUT_PROP_SEMI_MT is not set, the device is assumed to be a true MT
 device.
 
+INPUT_PROP_TOPBUTTONPAD:
+-----------------------
+Some laptops, most notably the Lenovo *40 series provide a trackstick
+device but do not have physical buttons associated with the trackstick
+device. Instead, the top area of the touchpad is marked to show
+visual/haptic areas for left, middle, right buttons intended to be used
+with the trackstick.
+
+If INPUT_PROP_TOPBUTTONPAD is set, userspace should emulate buttons
+accordingly. This property does not affect kernel behavior.
+The kernel does not provide button emulation for such devices but treats
+them as any other INPUT_PROP_BUTTONPAD device.
+
 Guidelines:
 ==========
 The guidelines below ensure proper single-touch and multi-finger functionality.