Add WEBROOT_REDIRECT documentation

Closes #802

docs/configuration.rst

@@ -71,6 +71,14 @@ Web settings
 
 The ``WEB_ADMIN`` contains the path to the main admin interface, while
 ``WEB_WEBMAIL`` contains the path to the Web email client.
+The ``WEBROOT_REDIRECT`` redirects all non-found queries to the set path.
+An empty ``WEBROOT_REDIRECT`` value disables redirecting and enables classic
+behavior of a 404 result when not found.
+All three options need a leading slash (``/``) to work.
+
+  .. note:: ``WEBROOT_REDIRECT`` has to point to a valid path on the webserver.
+    This means it cannot point to any services which are not enabled.
+    For example, don't point it to ``/webmail`` when ``WEBMAIL=none``
 
 Both ``SITENAME`` and ``WEBSITE`` are customization options for the panel menu
 in the admin interface, while ``SITENAME`` is a customization option for