Spell out what this property means to userspace. If the property is set, all
directional axes must be accelerometer axes, any other axes are left as-is.
This allows an accelerometer device to e.g. have an ABS_WHEEL.

It is not permitted to mix normal directional axes and accelerometer axes on
the same device node.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Acked-by: Jiri Kosina <jkosina@suse.cz>
Reviewed-by: Bastien Nocera <hadess@hadess.net>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>