[master] 2c8aefc3f improve the vcl(label) documentation

[Nils Goroll]

commit 2c8aefc3fe9d6982a1a1d3939a7121abc95c13ea
Author: Nils Goroll <nils.goroll at uplex.de>
Date:   Wed Nov 13 12:12:37 2019 +0100

    improve the vcl(label) documentation

diff --git a/doc/sphinx/users-guide/vcl-built-in-subs.rst b/doc/sphinx/users-guide/vcl-built-in-subs.rst
index 810ffedad..421186e4e 100644
--- a/doc/sphinx/users-guide/vcl-built-in-subs.rst
+++ b/doc/sphinx/users-guide/vcl-built-in-subs.rst
@@ -107,8 +107,14 @@ of the following keywords:
     :ref:`vcl_hash` to :ref:`vcl_purge`.
 
   ``vcl(label)``
-    Switch to vcl labelled *label*. This will continue vcl processing
-    in this vcl's :ref:`vcl_recv` as if it was the active vcl.
+    Switch to vcl labelled *label*.
+
+    This will roll back the request as if ``std.rollback(req)`` was
+    called and continue vcl processing in :ref:`vcl_recv` of the vcl
+    labelled *label* as if it was the active vcl.
+
+    The ``vcl(label)`` return is only valid while the ``req.restarts``
+    count is zero and if used from the active vcl.
 
     See the :ref:`ref_cli_vcl_label` command.