Better documentation of mcast_to_unicast option

This option is useful for Wifi bridge but need some tweak.

Document it from kernel patches documentation

Signed-off-by: Bastien Roucariès <rouca@debian.org>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>

man/man8/bridge.8

@@ -383,6 +383,32 @@ there is no MDB entry. By default this flag is on.
 Controls whether a given port will replicate packets using unicast
 instead of multicast. By default this flag is off.
 
+This is done by copying the packet per host and
+changing the multicast destination MAC to a unicast one accordingly.
+
+.BR mcast_to_unicast
+works on top of the multicast snooping feature of
+the bridge. Which means unicast copies are only delivered to hosts which
+are interested in it and signalized this via IGMP/MLD reports
+previously.
+
+This feature is intended for interface types which have a more reliable
+and/or efficient way to deliver unicast packets than broadcast ones
+(e.g. WiFi).
+
+However, it should only be enabled on interfaces where no IGMPv2/MLDv1
+report suppression takes place. IGMP/MLD report suppression issue is usually
+overcome by the network daemon (supplicant) enabling AP isolation and
+by that separating all STAs.
+
+Delivery of STA-to-STA IP mulitcast is made possible again by
+enabling and utilizing the bridge hairpin mode, which considers the
+incoming port as a potential outgoing port, too (see
+.B hairpin
+option).
+Hairpin mode is performed after multicast snooping, therefore leading to
+only deliver reports to STAs running a multicast router.
+
 .TP
 .BR "neigh_suppress on " or " neigh_suppress off "
 Controls whether neigh discovery (arp and nd) proxy and suppression is