doc: add AF_XDP queue setup information

doc: add AF_XDP queue setup information

When an AF_XDP PMD is created without specifying the 'start_queue', the
default Rx queue associated with the socket will be Rx queue 0. A common
scenario encountered by users new to AF_XDP is that they create the
socket on queue 0 however their interface is configured with many more
queues. In this case, traffic might land on for example queue 18 which
means it will never reach the socket.

This commit updates the AF_XDP documentation with instructions on how to
configure the interface to ensure the traffic will land on queue 0 and
thus reach the socket successfully.

Signed-off-by: Ciara Loftus <ciara.loftus@intel.com>
Acked-by: Ferruh Yigit <ferruh.yigit@intel.com>