feat(setup/advanced): document PUT /v1/portforward (#126)

Author: jagaimoworks

setup/advanced/control-server.md

@@ -97,6 +97,7 @@ The HTTP control server allows to modify the state of OpenVPN or Wireguard.
 ## Port forwarding
 
 - HTTP GET to `/v1/portforward` to obtain the port forwarded such as `{"port":5914}`
+- HTTP PUT to `/v1/portforwarded` with a body `{"ports": [5914]}` which overrides the exising port forwardings with the provided list of ports, thus also allowing clearing said portforwardings with `{"ports": []}`
 
 ## DNS
 

setup/advanced/vpn-port-forwarding.md

@@ -51,7 +51,9 @@ Thanks to [@Marsu31](https://github.com/Marsu31)
 
 ## Allow a forwarded port through the firewall
 
-For non-native integrations where you have a designated forwarded port from your VPN provider, you can allow it by adding it to the environment variable `FIREWALL_VPN_INPUT_PORTS`.
+For non-native integrations where you have a designated forwarded port from your VPN provider, you can allow it by adding it to the environment variable `FIREWALL_VPN_INPUT_PORTS`. This allows you to statically set your port forwardings whenever Gluetun starts.
+
+In some cases static port forwardings may not suffice, which is why Gluetun's control server provides an approrpiate endpoint to be able to set or unset your forwarded ports during runtime, giving you the ability to implement your own custom solution. See [the control server port forwarding section](control-server.md#port-forwarding).
 
 ## Port redirection using iptables