ethdev: document new error code for reset

ethdev: document new error code for reset

When VF reset will be supported by drivers, the API will most likely
have to return -EAGAIN to avoid blocking when the VF cannot be reset
because the PF is down.
Document it immediately even if it's not yet supported, so that users
and developers can already take into account about this use case, and
thus avoid an API-incompatible change later on.

This is based on real-world production usage and customer escalations,
using earlier patches from Intel [1].

[1]
http://dpdk.org/ml/archives/dev/2017-October/079692.html

Signed-off-by: Luca Boccassi <bluca@debian.org>
Acked-by: Thomas Monjalon <thomas@monjalon.net>