ipv6: document ndisc_notify in networking/ip-sysctl.txt

I slipped in a new sysctl without proper documentation. I would like to
make up for this now.
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/ip-sysctl.txt

@@ -1331,6 +1331,12 @@ force_tllao - BOOLEAN
 	race condition where the sender deletes the cached link-layer address
 	prior to receiving a response to a previous solicitation."
 
+ndisc_notify - BOOLEAN
+	Define mode for notification of address and device changes.
+	0 - (default): do nothing
+	1 - Generate unsolicited neighbour advertisements when device is brought
+	    up or hardware address changes.
+
 icmp/*:
 ratelimit - INTEGER
 	Limit the maximal rates for sending ICMPv6 packets.