doc: announce crypto structures rework

doc: announce crypto structures rework

The current crypto operation and symmetric crypto operation
structures will be reworked for correctness and improvement,
reducing also their sizes, to fit into less cache lines,
as stated in the following RFC:

http://dpdk.org/dev/patchwork/patch/24011/

Signed-off-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>
Acked-by: John McNamara <john.mcnamara@intel.com>
Acked-by: Declan Doherty <declan.doherty@intel.com>
Acked-by: Fiona Trahe <fiona.trahe@intel.com>
Acked-by: Hemant Agrawal <hemant.agrawal@nxp.com>
Acked-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>