man: ip-tunnel.8: Document missing 6rd action

Also drop the non-terminal 'TIME' description as it is not referenced
anywhere.
Signed-off-by: Phil Sutter <phil@nwl.cc>

man/man8/ip-tunnel.8

@@ -11,7 +11,7 @@ ip-tunnel - tunnel configuration
 .ti -8
 .BR "ip "
 .RI "[ " OPTIONS " ]"
-.BR "tunnel" " { " add " | " change " | " del " | " show " | " prl " }"
+.BR "tunnel" " { " add " | " change " | " del " | " show " | " prl " | " 6rd " }"
 .RI "[ " NAME " ]"
 .br
 .RB "[ " mode
@@ -42,6 +42,12 @@ ip-tunnel - tunnel configuration
 .B prl-delete
 .IR ADDR " ]"
 .br
+.RB "[ " 6rd-prefix
+.IR ADDR " ] ["
+.B 6rd-relay_prefix
+.IR ADDR " ] [
+.BR 6rd-reset " ]"
+.br
 .RB "[ [" no "]" pmtudisc " ]"
 .RB "[ " dev
 .IR PHYS_DEV " ]"
@@ -75,9 +81,6 @@ ip-tunnel - tunnel configuration
 .ti -8
 .IR KEY " := { " DOTTED_QUAD " | " NUMBER " }"
 
-.ti -8
-.IR TIME " := " NUMBER "[s|ms]"
-
 .SH DESCRIPTION
 .B tunnel
 objects are tunnels, encapsulating packets in IP packets and then