doc: announce moving ethdev actions to general modify

doc: announce moving ethdev actions to general modify

Currently there is a dedicated modify action for each
packet field that the application wants to change.
For example:
RTE_FLOW_ACTION_TYPE_SET_IPV4_DST to modify destination of IPv4.

A new action RTE_FLOW_ACTION_TYPE_MODIFY_FIELD added the ability
to use the same action to modify any field, in addition to be able to
modify the value based on different field and not just immediate value.

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: Jerin Jacob <jerinj@marvell.com>