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".

Does it worth adding a comment helping future developers 
understand what's going on?

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>

---

diff --git a/include/net/neighbour.h b/include/net/neighbour.h
index 09f9fc6..bc34144 100644
--- a/include/net/neighbour.h
+++ b/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)