man: add short blurb about InputClass configuration in servers 1.8
man: add short blurb about InputClass configuration in servers 1.8

Signed-off-by: Peter Hutterer <b9c50af7afa7642f6f854434cda9ffe54ece8b85@who-t.net>
(cherry picked from commit 5f546777173dc78146025499d186fe97d31eaeac)

philipn/xserver-xorg-input-synaptics,philipn/xserver-xorg-input-synaptics,philipn/xserver-xorg-input-synaptics,RsrchBoy/xserver-xorg-input-synaptics,RsrchBoy/xserver-xorg-input-synaptics