Based on the discussion here: https://forum.vyos.io/t/dynamic-dns-not-wollowing-web-options/12309 it seems necessary to note that setting the web-options on a given interface is not sufficient for determining the IP address when behind NAT.
I've added some additional detail, which I think will make that more clear, as well as listed the commands as required to set up DDNS behind NAT.
Further I updated the section on RFC2136 to accurately show address instead of interface
Updated command syntax for dynamic dns - changed set service dns dynamic interface to set service dns dynamic address.
Changed the login option from 'login' to 'username'
Changed the web options from 'use-web' to 'web-options'
Changed because I ran into the command syntax change on a 1.4 install. Updating documents to match.
Add documentation for "set service dns forwarding source-address <address>"
We needed this setting in a split-horizon DNS down a VPN tunnel to force the source address to a an IP that can be routed back instead of the VPN tunnel endpoint, but can only find it described in the command line currently.
