doc: announce more fields in flow API for VLAN

doc: announce more fields in flow API for VLAN

Structs rte_flow_item_eth and rte_flow_item_vlan will be modified,
to include an additional value, indicating existence or absence of VLAN
headers following the current header, as proposed in RFC
https://mails.dpdk.org/archives/dev/2020-August/177536.html.
Because of ABI break this change is proposed for 20.11.

Signed-off-by: Dekel Peled <dekelp@mellanox.com>
Acked-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Andrew Rybchenko <arybchenko@solarflare.com>
Acked-by: Ajit Khaparde <ajit.khaparde@broadcom.com>
Acked-by: Ori Kam <orika@mellanox.com>