doc: announce ethdev port id range increase

doc: announce ethdev port id range increase

This is an API/ABI change notice for DPDK 17.11 announcing the redefinition
of port_id. port_id is currently defined as uint8_t, which is limited to
the range 0 to 255. A larger range is required for vdev scalability.

It is necessary for a redefinition of port_id to extend it from 1 bytes to
2 bytes. All ethdev APIs and usages related to port_id will be changed at
the same time.

Signed-off-by: Zhiyong Yang <zhiyong.yang@intel.com>
Acked-by: Jianfeng Tan <jianfeng.tan@intel.com>
Acked-by: Yuanhan Liu <yliu@fridaylinux.org>
Acked-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>
Acked-by: Remy Horton <remy.horton@intel.com>
Acked-by: Shahaf Shuler <shahafs@mellanox.com>