doc: guarantee end sequence for continuous scroll

doc: guarantee end sequence for continuous scroll

GTK handles LIBINPUT_EVENT_POINTER_SCROLL_CONTINUOUS as
GDK_SCROLL_SMOOTH, the same event type that is used to handle
LIBINPUT_EVENT_POINTER_SCROLL_FINGER.

Because Mutter and other compositors, like wlroots based compositors,
translate libinput terminating event to axis_stop instead of doing their
own emulation, if libinput stops sending terminating events, it will
cause client bugs.

Since libinput always sends the terminating event for trackpoints and
button scrolling and there are even tests in place to check for them,
update the documentation to guarantee the terminating scroll sequence.

Signed-off-by: José Expósito <jose.exposito89@gmail.com>