qemu-doc: The "-net nic" option can be used with "netdev=...", too

Looks like we missed to document that it is also possible to specify
a netdev with "-net nic" - which is very useful if you want to
configure your on-board NIC to use a backend that has been specified
with "-netdev".

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>