doc: announce hiding crypto session structures

doc: announce hiding crypto session structures

The structures rte_cryptodev_sym_session and
rte_cryptodev_asym_session are not used by the
application directly. The application just need
an opaque pointer which it can attach to rte_crypto_op
while enqueue.
Hence, these structures can be internal to library
hidden from the user.

Signed-off-by: Akhil Goyal <gakhil@marvell.com>
Acked-by: Anoob Joseph <anoobj@marvell.com>
Acked-by: Jerin Jacob <jerinj@marvell.com>
Acked-by: Matan Azrad <matan@nvidia.com>