Input: alps - document separate pointstick button bits for V2 devices

Non interleaved dualpoint v2 devices have separate pointstick button bits,
document this.
Signed-off-by: NHans de Goede <hdegoede@redhat.com>
Signed-off-by: NDmitry Torokhov <dmitry.torokhov@gmail.com>

Documentation/input/alps.txt

@@ -94,6 +94,10 @@ PS/2 packet format
 
 Note that the device never signals overflow condition.
 
+For protocol version 2 devices when the trackpoint is used, and no fingers
+are on the touchpad, the M R L bits signal the combined status of both the
+pointingstick and touchpad buttons.
+
 ALPS Absolute Mode - Protocol Version 1
 --------------------------------------
 
@@ -107,7 +111,7 @@ ALPS Absolute Mode - Protocol Version 1
 ALPS Absolute Mode - Protocol Version 2
 ---------------------------------------
 
- byte 0:  1    ?    ?    ?    1    ?    ?    ?
+ byte 0:  1    ?    ?    ?    1  PSM  PSR  PSL
  byte 1:  0   x6   x5   x4   x3   x2   x1   x0
  byte 2:  0  x10   x9   x8   x7    ?  fin  ges
  byte 3:  0   y9   y8   y7    1    M    R    L
@@ -115,7 +119,8 @@ ALPS Absolute Mode - Protocol Version 2
  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.
+the DualPoint Stick. For non interleaved dualpoint devices the pointingstick
+buttons get reported separately in the PSM, PSR and PSL bits.
 
 Dualpoint device -- interleaved packet format
 ---------------------------------------------