[NEIGH]: Add a comment describing what a NUD stands for.

When I studied the neighbor code I puzzled over what the NUD can mean
for quite a long time.

Finally I asked Alexey and he said that this was smth like "neighbor
unreachability detection".
Signed-off-by: NPavel Emelyanov <xemul@openvz.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/net/neighbour.h

@@ -26,6 +26,10 @@
 #include <linux/sysctl.h>
 #include <net/rtnetlink.h>
 
+/*
+ * NUD stands for "neighbor unreachability detection"
+ */
+
 #define NUD_IN_TIMER	(NUD_INCOMPLETE|NUD_REACHABLE|NUD_DELAY|NUD_PROBE)
 #define NUD_VALID	(NUD_PERMANENT|NUD_NOARP|NUD_REACHABLE|NUD_PROBE|NUD_STALE|NUD_DELAY)
 #define NUD_CONNECTED	(NUD_PERMANENT|NUD_NOARP|NUD_REACHABLE)