Documentation: explain the 2 main viable event handling approaches

Stop pretending that having a separate event handling thread is a bad thing,
specifically delete the "[this] option is not very nice either, but may be
the nicest option available to you if the "proper" approach can not be applied
to your application", which suggests that using poll integration into a main
loop is the one and only "proper" approach.

Instead clearly document there are 2 viable approaches, using a separate
thread, or poll integration into a main loop.

Also stop claiming that libusb does not use threads internally, as with
the new hotplug support this is no longer true.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Documentation: add an Using an event handling thread section

Signed-off-by: Hans de Goede <hdegoede@redhat.com>