man: ip-neighbour.8: Document all known nud states

Not sure how useful they are in practice, but as 'ip neigh' supports
setting them all, they deserve to be described as well.

While at it, also add a missing layer of indentation to the subordinate
nud state list.

Signed-off-by: Phil Sutter <phil@nwl.cc>

man/man8/ip-neighbour.8

@@ -35,7 +35,8 @@ ip-neighbour \- neighbour/arp tables management.
 
 .ti -8
 .IR STATE " := {"
-.BR permanent " | " noarp " | " stale " | " reachable " }"
+.BR permanent " | " noarp " | " stale " | " reachable " | " none " |"
+.BR incomplete " | " delay " | " probe " | " failed " }"
 
 .SH DESCRIPTION
 The
@@ -86,6 +87,7 @@ the state of the neighbour entry.
 is an abbreviation for 'Neighbour Unreachability Detection'.
 The state can take one of the following values:
 
+.RS
 .TP
 .B permanent
 the neighbour entry is valid forever and can be only
@@ -105,6 +107,24 @@ This option to
 .B ip neigh
 does not change the neighbour state if it was valid and the address
 is not changed by this command.
+.TP
+.B none
+this is a pseudo state used when initially creating a neighbour entry or after
+trying to remove it before it becomes free to do so.
+.TP
+.B incomplete
+the neighbour entry has not (yet) been validated/resolved.
+.TP
+.B delay
+neighbor entry validation is currently delayed.
+.TP
+.B probe
+neighbor is being probed.
+.TP
+.B failed
+max number of probes exceeded without success, neighbor validation has
+ultimately failed.
+.RE
 .RE
 
 .TP