Document HTTP proxy tunnelling option (#9054)

Closes #7209. Documents CURLOPT_HTTPPROXYTUNNEL with a data/config.php example for HTTPS feeds through HTTP CONNECT proxies.

Co-authored-by: Gerard Alvear <gerard.alvear@logiqd.me>

docs/en/admins/17_configs_not_ui.md

@@ -48,6 +48,23 @@ Example: `'<img class="logo" src="https://example.net/Hello.png" alt="Logo Examp
 
 See the documentation directly in the source code `config.default.php`
 
+#### System config: HTTP proxy tunnelling
+
+When FreshRSS fetches HTTPS feeds through an HTTP proxy that supports `CONNECT`,
+enable tunnelling in `data/config.php` by adding `CURLOPT_HTTPPROXYTUNNEL` to
+the global cURL options:
+
+```php
+'curl_options' => [
+	CURLOPT_PROXY => 'proxy.example.net:8080',
+	CURLOPT_PROXYTYPE => CURLPROXY_HTTP,
+	CURLOPT_HTTPPROXYTUNNEL => true,
+],
+```
+
+This is useful for proxies such as VPN gateway containers. The option applies
+to all feed requests unless overridden by a feed-specific configuration.
+
 ## Application wide constants
 
 See `./constants.php`. Do not edit this file. Create/edit `./constants.local.php` instead.