ethdev: document VMDq Rx configuration

ethdev: document VMDq Rx configuration

From documentation it is very unclear how VMDq configuration can be
tweaked, and online search offer very poor results.

This patch will ultimately spawn an online documentation page
for the rte_eth_vmdq_rx_conf struct which will eventually add a bit of
documentation about the rx_mode tag and how to allow e.g. VMDq pools
to receive packets without VLAN tags.

Signed-off-by: Tom Barbette <tom.barbette@ulg.ac.be>
Acked-by: John McNamara <john.mcnamara@intel.com>