Rewrite for 4.0 - might still need review

doc/sphinx/users-guide/vcl-syntax.rst

@@ -95,4 +95,5 @@ To call a subroutine, use the call keyword followed by the subroutine's name:
 call pipe_if_local;
 
 Varnish has quite a few built in subroutines that are called for each
-transaction as it flows through Varnish. See :ref:`vcl-built-in-subs`.
+transaction as it flows through Varnish. These builtin subroutines are all named vcl_*. Your own subroutines cannot start their name with vcl_.
+See :ref:`vcl-built-in-subs`.