client: Remove misplaced documentation about main loop intergration

There was documentation about how to integrate the display server file
descriptor in the documentation about wl_display_dispatch_pending().
This is not the right place to put it, and it also had incorrect usage
of the API (calling wl_display_dispatch_queue() on input on an unrelated
fd) as an example.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Daniel Stone <daniels@collabora.com>