doc: announce change to ethdev modify action data

doc: announce change to ethdev modify action data

In the current implementation,
the action rte_flow_action_modify_field is not well defined
for fields larger than 64 bits (for example IPv6 source)
In addition, the byte order is also not well defined.

Both of those issue should be fixed.

Signed-off-by: Ori Kam <orika@nvidia.com>
Acked-by: Matan Azrad <matan@nvidia.com>
Acked-by: Ajit Khaparde <ajit.khaparde@broadcom.com>
Acked-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
Acked-by: Jerin Jacob <jerinj@marvell.com>