doc/user: add a warning that udevadm test doesn't always work

We rely on the udev keyboard builtin to set the fuzz but that builtin isn't
run during udevadm test. So running sudo udevadm test shows the LIBINPUT_FUZZ
properties the first time round (still set from boot), but not the second time.

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