[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: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
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