Fix various NSEDoc problems.

Found by David Matousek.
http://seclists.org/nmap-dev/2013/q1/88

scripts/ip-forwarding.nse

@@ -24,6 +24,8 @@ to be on the LAN.
 -- @usage
 -- sudo nmap -sn <target> --script ip-forwarding --script-args='target=www.example.com'
 --
+-- @args ip-forwarding.target the host to which to attempt forwarding (required).
+--
 -- @output
 -- | ip-forwarding: 
 -- |_  The host has ip forwarding enabled, tried ping against (www.example.com)