doc: announce change in ethdev flow action port ID

doc: announce change in ethdev flow action port ID

By its very name, action PORT_ID means that packets hit an ethdev with the
given DPDK port ID. At least the current comments don't state the opposite.

However some drivers implement it in a different way and direct traffic to
the opposite end of the "wire" plugged to the given ethdev. For example in
the case of a VF representor traffic is redirected to the corresponding VF
itself rather than to the representor ethdev and OvS uses PORT_ID action
this way.

The documentation must be clarified and, likely, rte_flow_action_port_id
structure should be extended to support both meanings.

Signed-off-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
Acked-by: Ori Kam <orika@nvidia.com>
Acked-by: Ajit Khaparde <ajit.khaparde@broadcom.com>
Acked-by: Thomas Monjalon <thomas@monjalon.net>