doc: announce extension of crypto data-unit length

doc: announce extension of crypto data-unit length

The struct member dataunit_len is introduced in DPDK 21.05.
It is limited to 16 bits to fit a padding hole in 32-bit build.
This means the maximum data-unit length is 64 KB.
Some use cases may benefit of a bigger size as the proposed 32 bits.

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Akhil Goyal <gakhil@marvell.com>
Acked-by: Ajit Khaparde <ajit.khaparde@broadcom.com>
Acked-by: Matan Azrad <matan@nvidia.com>