public/io/netif.h: document control ring and toeplitz hashing

public/io/netif.h: document control ring and toeplitz hashing

This patch documents a new shared ring between frontend and backend that
can be used to pass bulk out-of-band data, such as that required to
implement toeplitz hashing in the backend such that it is configurable by
the frontend (which is needed to support NDIS RSS for Windows guests).

The patch then goes on to document the messages passed over the control
ring that can be used to configure toeplitz hashing and a new extra info
fragment that can be used to pass hash values between frontend and
backend for both transmit and receive packets.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Jan Beulich <jbeulich@suse.com>
Cc: Keir Fraser <keir@xen.org>
Cc: Tim Deegan <tim@xen.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>