public/io/netif.h: document the reality of netif_rx_request/reponse

public/io/netif.h: document the reality of netif_rx_request/reponse

Because GSO metadata is passed from backend to frontend using
netif_extra_info segments, which do not carry information stating which
netif_rx_request_t was consumed to free up their slot, frontends must
assume some form of identity relation between ring slot and request.
Hence, so that it is able to use GSO metadata, Linux netfront simply
assumes rx responses appear in the same ring slot as their corresponding
request.

This patch documents the assumption made by Linux netfront and the
necessity of the assumption (to support GSO) so that backends are coded
to be compatible.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>