timer: change the timer offset warning to a client bug

timer: change the timer offset warning to a client bug

This looks like a libinput bug (well, it does say "libinput bug" on the
package) but it hasn't been that for a long time. The cause is almost always
insufficient motivation to call libinput_dispatch() by the caller, or at least
not doing it with the celerity libinput demands (and deserves, if I may say
so).

Up-, down- or side-grade it to a client bug, so the outrage can be
directed elsewhere, preferably away from me. And add a section to the
documentation, just in case someone actually reads this stuff.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Olivier Fourdan <ofourdan@redhat.com>