iproute2: add ip-l2tp man page

Add a man page to cover the "ip l2tp" commands. Add a reference to it
in the main ip page.

This version removes the unnecessary setting of promiscuous mode
in the examples.
Signed-off-by: James Chapman <jchapman@katalix.com>

man/man8/ip.8

@@ -113,7 +113,7 @@ host addresses.
 
 .TP
 .B l2tp
-- tunnel PPP over IP (L2TP).
+- tunnel ethernet over IP (L2TPv3).
 
 .TP
 .B link
@@ -205,6 +205,7 @@ was written by Alexey N. Kuznetsov and added in Linux 2.2.
 .SH SEE ALSO
 .BR ip-address (8),
 .BR ip-addrlabel (8),
+.BR ip-l2tp (8),
 .BR ip-link (8),
 .BR ip-maddress (8),
 .BR ip-monitor (8),