This should be bound only to mouse buttons 4, 5, 6, and 7 on
non-Windows systems.
-An optional prefix ARG can be used to change the step of horizontal
-scrolling. The arg numeric value can be typed before starting to scroll.
-The value is saved in the variable `mouse-wheel-scroll-amount-horizontal'."
+Optional argument ARG (interactively, prefix numeric argument) controls
+the step of horizontal scrolling.
+
+The variable `mouse-wheel-scroll-amount-horizontal' records the last
+value of ARG, and the command uses it in subsequent scrolls."
(interactive (list last-input-event current-prefix-arg))
(let* ((selected-window (selected-window))
(scroll-window (mouse-wheel--get-scroll-window event))