doc: announce deprecation of port mirroring API

doc: announce deprecation of port mirroring API

A new API is planned to be introduced for sampling and mirroring
with rte_flow. It should be more generic and allow more use cases.

This deprecation is to show the direction, avoiding overlapping APIs.

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Jerin Jacob <jerinj@marvell.com>
Acked-by: Andrew Rybchenko <arybchenko@solarflare.com>
Acked-by: Ajit Khaparde <ajit.khaparde@broadcom.com>
Acked-by: David Marchand <david.marchand@redhat.com>