public/io/netif.h: move and amend multicast control documentation

public/io/netif.h: move and amend multicast control documentation

netif.h contains a specification of the XEN_NETIF_EXTRA_TYPE_MCAST_{ADD,DEL}
extra info messages require to manipulate a multicast filter list maintained
by a backend and specifies the xenstore negotiation protocol in a comment
just above the structure defintion, which is easy to miss.

This patch moves the documentation of the xenstore negotiation to be
co-located with the documentation for other features and also amends the
wording to be clearer.

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: Wei Liu <wei.liu2@citrix.com>
[ ijc -- added a blank line to the comment ]