doc: announce new ethdev field in Rx queue info

doc: announce new ethdev field in Rx queue info

Struct rte_eth_rxq_info will be modified to include a new field, indicating
the size of each buffer that could be used for HW to receive packets. Add
this field to rte_eth_rxq_info to expose relevant information to upper
layer users/application.

For more details:
https://mails.dpdk.org/archives/dev/2020-July/176135.html

Signed-off-by: Chengchang Tang <tangchengchang@huawei.com>
Acked-by: Andrew Rybchenko <arybchenko@solarflare.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>
Acked-by: Ferruh Yigit <ferruh.yigit@intel.com>