docs: recommend SCRIPT_NAME=/subfolder

Author: pajod

docs/source/faq.rst

@@ -13,8 +13,12 @@ How do I set SCRIPT_NAME?
 By default ``SCRIPT_NAME`` is an empty string. The value could be set by
 setting ``SCRIPT_NAME`` in the environment or as an HTTP header. Note that
 this headers contains and underscore, so it is only accepted from trusted
-forwarders listed in the ``forwarded-allow-ips`` setting.
+forwarders listed in the :ref:`forwarded-allow-ips` setting.
 
+.. note::
+
+   If your application should appear in a subfolder, your ``SCRIPT_NAME``
+   would typically start with single slash but contain no trailing slash.
 
 Server Stuff
 ============