protocol: minor clarification for axis_discrete events

Explicitly spell out that multiple axis_discrete events *for the same axis*
within the same wl_pointer.frame are not permitted.

References: https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/911#note_1438099

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
1 file changed