doc: announce ethdev flag and field for shared queue

doc: announce ethdev flag and field for shared queue

To support shared Rx queue, this patch announces new offload flag
RTE_ETH_RX_OFFLOAD_SHARED_RXQ and new shared_group field to struct
rte_eth_rxconf in DPDK v21.11.

[1] mail list discussion:
https://mails.dpdk.org/archives/dev/2021-July/215575.html

Signed-off-by: Xueming Li <xuemingl@nvidia.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>
Acked-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
Acked-by: Jerin Jacob <jerinj@marvell.com>