doc: announce ABI change for Tx offload

doc: announce ABI change for Tx offload

This is an ABI change notice for DPDK 17.08 in ethdev
about changes in rte_eth_txmode structure.

Currently Tx offloads are enabled by default, and can be disabled
using ETH_TXQ_FLAGS_NO* flags. This behaviour is not consistent with
the Rx side where the Rx offloads are disabled by default and enabled
according to bit field in rte_eth_rxmode structure.

The proposal is to disable the Tx offloads by default, and provide
a way for the application to enable them in rte_eth_txmode structure.
Besides of making the Tx configuration API more consistent for
applications, PMDs will be able to provide a better out of the
box performance.
Finally, as part of the work, the ETH_TXQ_FLAGS_NO* will
be superseded as well.

Signed-off-by: Shahaf Shuler <shahafs@mellanox.com>
Acked-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>
Acked-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>