Currently, the documentation for NotifyPointerAxis() just mentions that
a smooth scroll event is emitted.
However, this is not entirely correct. For each NotifyPointerAxis(),
mutter emits an emulated discrete scrolling event based on the
submitted accumulated smooth scrolling deltas.
Additionally, it doesn't mention how the motion deltas need to be
interpreted.
So, document the NotifyPointerAxis() notification better by mentioning
the emulation of discrete scroll events, how these discrete scroll
events are calculated and how the motion deltas need to be interpreted.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1727>
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