doc: announce splitting ethdev ops struct

doc: announce splitting ethdev ops struct

For the ABI compatibility it is better to hide internal data structures
from the application as much as possible. But because of some inline
functions 'struct eth_dev_ops' can't be hidden completely.

Plan is to split the 'struct eth_dev_ops' into two as ones used by
inline functions and ones not used, and hide the second part that not
used by inline functions completely to the application.

Because of ABI break the work will be done in 20.11

Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>
Acked-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Andrew Rybchenko <arybchenko@solarflare.com>
Acked-by: David Marchand <david.marchand@redhat.com>