doc: announce behaviour change to i40e RSS
doc: announce behaviour change to i40e RSS

Add deprecation notice to highlight a future change in the
default configuration behavior for flexible payload for RSS.

Signed-off-by: John McNamara <af31595c36a4239b9676e9b3a777d3e2f15a6c4d@intel.com>
Acked-by: Andrey Chilikin <e7d64155883734f893a62d2f3554768227530801@intel.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk