Input: document the units for resolution of size axes

Today, the resolution of size axes is not documented. As a result, it's
not clear what the canonical interpretation of this value should be. On
Android, there is a need to calculate the size of the touch ellipse in
physical units (millimeters).

After reviewing linux source, it turned out that most of the existing
usages are already interpreting this value as "units/mm". This
documentation will make it explicit. This will help device
implementations with correctly following the linux specs, and will
ensure that the devices will work on Android without needing further
customized parameters for scaling of major/minor values.

Signed-off-by: Siarhei Vishniakou <svv@google.com>
Reviewed-by: Jeff LaBundy <jeff@labundy.com>
Link: https://lore.kernel.org/r/20220520084514.3451193-1-svv@google.com
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

include/uapi/linux/input.h

@@ -78,10 +78,13 @@ struct input_id {
  * Note that input core does not clamp reported values to the
  * [minimum, maximum] limits, such task is left to userspace.
  *
- * The default resolution for main axes (ABS_X, ABS_Y, ABS_Z)
- * is reported in units per millimeter (units/mm), resolution
- * for rotational axes (ABS_RX, ABS_RY, ABS_RZ) is reported
- * in units per radian.
+ * The default resolution for main axes (ABS_X, ABS_Y, ABS_Z,
+ * ABS_MT_POSITION_X, ABS_MT_POSITION_Y) is reported in units
+ * per millimeter (units/mm), resolution for rotational axes
+ * (ABS_RX, ABS_RY, ABS_RZ) is reported in units per radian.
+ * The resolution for the size axes (ABS_MT_TOUCH_MAJOR,
+ * ABS_MT_TOUCH_MINOR, ABS_MT_WIDTH_MAJOR, ABS_MT_WIDTH_MINOR)
+ * is reported in units per millimeter (units/mm).
  * When INPUT_PROP_ACCELEROMETER is set the resolution changes.
  * The main axes (ABS_X, ABS_Y, ABS_Z) are then reported in
  * units per g (units/g) and in units per degree per second