man: add a warning that wheel emu inertia must be set and it isn't inertia

First, it's not actually inertia, it's simply the scroll distance, yay for the
misnomer.

And it needs to be set for any device that is more fine-grained than a
mouse, especially absolute devices. For example the VirtualBox device has an
abs max of 32767, so a simple motion may have a delta of to 2000 units and
that results in 200 scroll events. That's a bit excessive.

Related to: https://bugs.freedesktop.org/show_bug.cgi?id=93617

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

man/evdev.man

@@ -125,6 +125,19 @@ Property: "Evdev Wheel Emulation Button".
 Specifies how far (in pixels) the pointer must move to generate button
 press/release events in wheel emulation mode.  Default: 10. Property: "Evdev
 Wheel Emulation Inertia".
+.IP
+This value must be set for any device does not resemble a standard mouse.
+Specifically, on absolute devices such as tablets the value should be set to
+a reasonable fraction of the expected movement to avoid excess scroll events.
+.IP
+.B WARNING:
+the name \*qinertia\*q is a misnomer. This option defines the distance
+required to generate one scroll event similar to the
+.B VertScrollDelta
+and
+.B HorizScrollDelta
+options. It does not enable inertia in the
+physical sense, scrolling stops immediately once the movement has stopped.
 .TP 7
 .BI "Option \*qEmulateWheelTimeout\*q \*q" integer \*q
 Specifies the time in milliseconds the