doc: announce flow API matching structs changes

doc: announce flow API matching structs changes

Proposing to replace protocol header fields in the ``rte_flow_item_*``
structures with the protocol structs.

This is both for documenting the intention and to be sure
``rte_flow_item_*`` always starts with complete protocol header.

Change will be done in two steps, at first step in v21.02 release,
protocol header struct will be added as union, for example:

Current ``struct rte_flow_item_eth``,

struct rte_flow_item_eth {
	struct rte_ether_addr dst;
	struct rte_ether_addr src;
	rte_be16_t type;
	uint32_t has_vlan:1;
	uint32_t reserved:31;
}

will become in v21.02:

__extension__
struct rte_flow_item_eth {
	union {
		struct {
			struct rte_ether_addr dst;
			struct rte_ether_addr src;
			rte_be16_t type;
		};
		struct rte_ether_hdr hdr;
	};
	uint32_t has_vlan:1;
	uint32_t reserved:31;
}

After this point usage should switch to 'hdr' struct.

And in the second step, in the v21.11 LTS release the protocol fields
will be removed, and the struct will become:

struct rte_flow_item_eth {
	struct rte_ether_hdr hdr;
	uint32_t has_vlan:1;
	uint32_t reserved:31;
}

Already many ``rte_flow_item_*`` structures implemented to have protocol
struct, target is convert all to this usage.

Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>
Acked-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
Acked-by: Ajit Khaparde <ajit.khaparde@broadcom.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Thomas Monjalon <thomas@monjalon.net>