Add a note that X-Forwared-Prefix should be configured as trusted header.

Author: jbtronics

deployment/proxies.rst

@@ -170,7 +170,8 @@ This is required for Symfony to generate the correct URLs.
 This can normally be configured in your reverse proxy configuration.
 If the application is served via the reverse proxy under a URL like
 ``https://localhost/app/``, then ``X-Forwarded-Prefix`` must be set to ``/app/`` by the
-reverse proxy.
+reverse proxy. Be sure that ``X-Forwared-Prefix`` is configured as trusted header, if
+you want to use this feature.
 
 Custom Headers When Using a Reverse Proxy
 -----------------------------------------