Input: alps - document stick behavior for protocol V2

Document that protocol V2 uses standard (bare) PS/2 mouse packets for the
DualPoint stick.
Signed-off-by: NHans de Goede <hdegoede@redhat.com>
Acked-By: NPali Rohár <pali.rohar@gmail.com>
Signed-off-by: NDmitry Torokhov <dmitry.torokhov@gmail.com>

Documentation/input/alps.txt

@@ -114,6 +114,9 @@ ALPS Absolute Mode - Protocol Version 2
  byte 4:  0   y6   y5   y4   y3   y2   y1   y0
  byte 5:  0   z6   z5   z4   z3   z2   z1   z0
 
+Protocol Version 2 DualPoint devices send standard PS/2 mouse packets for
+the DualPoint Stick.
+
 Dualpoint device -- interleaved packet format
 ---------------------------------------------
 
@@ -127,6 +130,11 @@ Dualpoint device -- interleaved packet format
  byte 7:    0   y6   y5   y4   y3   y2   y1   y0
  byte 8:    0   z6   z5   z4   z3   z2   z1   z0
 
+Devices which use the interleaving format normally send standard PS/2 mouse
+packets for the DualPoint Stick + ALPS Absolute Mode packets for the
+touchpad, switching to the interleaved packet format when both the stick and
+the touchpad are used at the same time.
+
 ALPS Absolute Mode - Protocol Version 3
 ---------------------------------------