doc: announce removal of ethdev flow director API

doc: announce removal of ethdev flow director API

The flow director config, part of the legacy filtering API,
was marked as deprecated last year.
A separate notice is added to make clear that these specific structs
will be removed as well in DPDK 20.11, as the rest of the legacy
filtering API.

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