doc/network_forwards: Split configuration into own table

Signed-off-by: Stéphane Graber <[email protected]>
Sponsored-by: ActivePort (https://www.activeport.com.au)

Author: stgraber

doc/howto/network_forwards.md

@@ -38,9 +38,18 @@ Property         | Type       | Required | Description
 :--              | :--        | :--      | :--
 `listen_address` | string     | yes      | IP address to listen on
 `description`    | string     | no       | Description of the network forward
-`config`         | string set | no       | Configuration options as key/value pairs (only `target_address` and `user.*` custom keys supported)
+`config`         | string set | no       | See table below
 `ports`          | port list  | no       | List of {ref}`port specifications <network-forwards-port-specifications>`
 
+### Forward configuration
+
+Network forwards have the following configuration options:
+
+Key                 | Type      | Default | Description
+:--                 | :---      | :------ | :----------
+`user.*`            | string    | -       | User defined key/value configuration
+`target_address`    | string    | -       | Default target address for anything not covered through a port definition
+
 (network-forwards-listen-addresses)=
 ### Requirements for listen addresses