doc: announce ABI change for security session

doc: announce ABI change for security session

Add 'uint64_t opaque_data' inside struct rte_security_session.
That allows upper layer to easily associate some user defined
data with the session.
Proposed new layout for:
struct rte_security_session {
	void *sess_private_data;
	/**< Private session material */
+	uint64_t opaque_data;
+	/**< Opaque user defined data */
};

Signed-off-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
Acked-by: Mohammad Abdul Awal <mohammad.abdul.awal@intel.com>
Acked-by: Fan Zhang <roy.fan.zhang@intel.com>
Acked-by: Akhil Goyal <akhil.goyal@nxp.com>