doc: announce ethdev API change for VF representors

doc: announce ethdev API change for VF representors

This is following the RFC being discussed and targets 18.05

http://dpdk.org/ml/archives/dev/2018-January/085716.html

Signed-off-by: Shahaf Shuler <shahafs@mellanox.com>
Acked-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Declan Doherty <declan.doherty@intel.com>
Acked-by: Remy Horton <remy.horton@intel.com>
Acked-by: Luca Boccassi <luca.boccassi@intl.att.com>
Acked-by: Alex Zelezniak <alexz@att.com>
Acked-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>