ripngd: adjust header for display command

Both rip and ripng can import routes from other protocols, e.g. ISIS.
But their header doesn't list the description for these abbreviations.

Adjust `show ipv6 ripng` 's header for display command.

Before:
```
Codes: R - RIPng, C - connected, S - Static, O - OSPF, B - BGP
Sub-codes:
```

After:
```
Codes: K - kernel route, C - connected, L - local, S - static,
       R - RIPng, O - OSPF, I - IS-IS, B - BGP, E - EIGRP, N - NHRP,
       T - Table, v - VNC, V - VNC-Direct, A - Babel, F - PBR,
       f - OpenFabric, t - Table-Direct
Sub-codes:
```

Signed-off-by: anlan_cs <[email protected]>

ripngd/ripngd.c

@@ -2070,7 +2070,10 @@ DEFUN (show_ipv6_ripng,
 
 	/* Header of display. */
 	vty_out(vty,
-		"Codes: R - RIPng, C - connected, S - Static, O - OSPF, B - BGP\n"
+		"Codes: K - kernel route, C - connected, L - local, S - static,\n"
+		"       R - RIPng, O - OSPF, I - IS-IS, B - BGP, E - EIGRP, N - NHRP,\n"
+		"       T - Table, v - VNC, V - VNC-Direct, A - Babel, F - PBR,\n"
+		"       f - OpenFabric, t - Table-Direct\n"
 		"Sub-codes:\n"
 		"      (n) - normal, (s) - static, (d) - default, (r) - redistribute,\n"
 		"      (i) - interface, (a/S) - aggregated/Suppressed\n\n"