doc: reduce initial offload API rework scope to drivers

doc: reduce initial offload API rework scope to drivers

Do ethdev new offloading API switch in two steps.

In v18.05 target is implementing the new ethdev-PMD offload interface,
which means converting all PMDs to new offloading API.

Next target is removing the old ethdev offload API.
It will effect applications and will force them to implement new
offloading API.

Fixes: 3004d3454192 ("doc: update deprecation of ethdev offload API")

Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>
Acked-by: Shahaf Shuler <shahafs@mellanox.com>