Document force-https.txt (#9460)

Closes #4616

The _Force HTTPS_ mechanism (`./force-https.default.txt` and `./data/force-https.txt`) was not documented. This adds a short section to the admin page about configuration without a user interface.

docs/en/admins/17_configs_not_ui.md

@@ -73,6 +73,21 @@ the global cURL options:
 This is useful for proxies such as VPN gateway containers. The option applies
 to all feed requests unless overridden by a feed-specific configuration.
 
+## Force HTTPS
+
+FreshRSS rewrites `http://` URLs to `https://` in feed content, for domains known to support HTTPS.
+The default list is defined in `./force-https.default.txt`; do not modify this file, but specify your own values in `./data/force-https.txt`, one domain per line:
+
+```text
+example.com
+www.example.net
+```
+
+Empty lines and comments starting with `#` are ignored.
+A listed domain also matches its sub-domains, e.g. `example.com` matches `www.example.com`.
+
+With Docker, this file is `force-https.txt` in the persistent data volume mounted at `/var/www/FreshRSS/data/`.
+
 ## Application wide constants
 
 See `./constants.php`. Do not edit this file. Create/edit `./constants.local.php` instead.