docs: net: document guidance of implementing the SR-IOV NDOs

docs: net: document guidance of implementing the SR-IOV NDOs

New drivers were prevented from adding ndo_set_vf_* callbacks
over the last few years. This was expected to result in broader
switchdev adoption, but seems to have had little effect.

Based on recent netdev meeting there is broad support for allowing
adding those ops.

There is a problem with the current API supporting a limited number
of VFs (100+, which is less than some modern HW supports).
We can try to solve it by adding similar functionality on devlink
ports, but that'd be another API variation to maintain.
So a netlink attribute reshuffling is a more likely outcome.

Document the guidance, make it clear that the API is frozen.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>