docs: formatdomain: Clarify configuration of iothread <-> virtqueue mapping

Add an example for the automatic/round-robin mapping of iothreads which
users should preferrably use. Until now the example contained even the
full mapping which could push users to use that instead.

Mention that the queues are then automatically distributed among the
iothreads.

Also clarify the need to set 'queues' when mapping threads explicitly
and how the queues are identified.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>