doc: announce old flow director API removal

doc: announce old flow director API removal

APIs for flow director filters has been replaced by rte_eth_dev_filter_ctrl
by previous releases. Enic, ixgbe and i40e are switched to support filter_ctrl
APIs, so the old APIs are useless, and ready to be removed now.
This patch announces the ABI change for these APIs removing.

Signed-off-by: Jingjing Wu <jingjing.wu@intel.com>
Acked-by: Helin Zhang <helin.zhang@intel.com>
Acked-by: Jijiang Liu <jijiang.liu@intel.com>
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>