doc: announce removal of vhost zero-copy dequeue

doc: announce removal of vhost zero-copy dequeue

Vhost-user dequeue zero-copy support will be removed in
20.11. The only known user is OVS where the feature is
still experimental, and has not received any update for
several years. This feature faces reliability issues and
is often conflicting with new features being implemented.

Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Acked-by: Chenbo Xia <chenbo.xia@intel.com>
Acked-by: Ian Stokes <ian.stokes@intel.com>
Acked-by: Ciara Loftus <ciara.loftus@intel.com>
Acked-by: Aaron Conole <aconole@redhat.com>