docs: networking: update XPS to account for netif_set_xps_queue

With the introduction of netif_set_xps_queue, XPS can be enabled
by the driver at initialization.

Update the documentation to reflect this, as otherwise users
may incorrectly believe that the feature is off by default.

Fixes: 537c00de ("net: Add functions netif_reset_xps_queue and netif_set_xps_queue")
Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>