Added an verbose example to X-FORWARDED-PREFIX documentation to make its function more clear.

Author: jbtronics

deployment/proxies.rst

@@ -165,14 +165,24 @@ Reverse proxy in a subpath / subfolder
 If you configure the reverse proxy in such a way that the Symfony application is
 mapped in a subpath / subfolder instead of the root path of the web host, you will need to
 pass the prefix to Symfony by setting the ``X-Forwarded-Prefix`` header.
-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. Be sure that ``X-Forwared-Prefix`` is configured as trusted header, if
+This is required for Symfony to generate the correct URLs. The header can normally be configured
+in your reverse proxy configuration. Be sure that ``X-Forwared-Prefix`` is configured as trusted header, if
 you want to use this feature.
 
+The ``X-Forwarded-Prefix`` is used by Symfony to prefix the base URL of request objects, which is
+used to generate absolute paths and URLs in Symfony applications. Without the header, the base URL would be only determined
+based on the configuration of the web server running Symfony, which leads to incorrect pathes/URLs, when the application
+is served under a subpath by a reverse proxy.
+
+For example if your symfony application is directly served under an URL like ``https://symfony.tld/`` 
+and you would like to use a reverse proxy to serve the application under ``https://public.tld/app/``, you would need
+to set the ``X-Forwarded-Prefix`` header to ``/app/`` in your reverse proxy configuration.
+Without the header, Symfony would generate URLs based on its servers base URL (e.g. ``/my/route``) instead of the correct 
+``/app/my/route``, which is required to access the route via the reverse proxy.
+
+The header can be different for each reverse proxy, so that access via different reverse proxies served under different
+subpaths can be handled correctly.
+
 Custom Headers When Using a Reverse Proxy
 -----------------------------------------