public/io/netif.h: clarifications to wire formats

public/io/netif.h: clarifications to wire formats

My previous patch 03809ae7 "document transmit and receive wire formats
separately" improved documentation of the receive and transmit wire
formats but further clarifications were requested.

This patch adds those clarifications.

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>