doc: refer to "the libinput documentation" in a few @see cases

This is in preparation for a change to sphinx as the user-visible
documentation. Ideally we could cross-link between the two but that's tricky
to do automatically. Linking to the html pages/anchors directly works fine but
risks the links going stale, especially while the documentation is still in
flux.

Having a generic "refer to the libinput documentation" is a bit of a cop-out
least this way the links cannot go stale.

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