doc: announce changes in security session struct

doc: announce changes in security session struct

The structure rte_security_session is not directly used
by the application. The application just need an opaque
pointer to attached to the mbuf or rte_crypto_op while
enqueue. Hence, it can be hidden inside the library
and would prevent unnecessary indirection to the priv
session data in fastpath.

Signed-off-by: Akhil Goyal <gakhil@marvell.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
Acked-by: Ajit Khaparde <ajit.khaparde@broadcom.com>
Acked-by: Anoob Joseph <anoobj@marvell.com>
Acked-by: Jerin Jacob <jerinj@marvell.com>