doc: explain virtio Rx/Tx functions

This patch explains current virtio PMD Rx/Tx callbacks, to help understand
what's the difference, and how to enable the right ones.

Signed-off-by: Zhihong Wang <zhihong.wang@intel.com>
Acked-by: John McNamara <john.mcnamara@intel.com>
Acked-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>