Added documentation about sub user_context_hash_url

Author: Nyholm

doc/varnish-configuration.rst

@@ -202,7 +202,10 @@ User Context
 
 Feature: :doc:`user context hashing <user-context>`
 
-Subroutines are provided in ``resources/config/varnish-[version]/fos_user_context.vcl``.
+Subroutines are provided in ``resources/config/varnish-[version]/fos_user_context.vcl`` and ``resources/config/varnish-[version]/fos_user_context_url.vcl``. The default lookup url is `/_fos_user_context_hash`. You 
+may change it by implementing your own ``user_context_hash_url`` subroutine. 
+
+
 To enable support add the following to ``your_varnish.vcl``: