doc: announce removal of L2 tunnel filtering API

doc: announce removal of L2 tunnel filtering API

The functions for L2 tunnel were missed when marking the legacy
filtering API as deprecated. That's why a separate notice is done
to make clear that it will be removed as well in DPDK 20.11.

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