[master] 7dc80a1c6 Simple addition on req.grace

[Dridi Boukelmoune]

commit 7dc80a1c6654b7edd9f4e5fade007c8531b9dcbe
Author: Pål Hermunn Johansen <hermunn at varnish-software.com>
Date:   Fri Sep 14 14:33:29 2018 +0200

    Simple addition on req.grace

diff --git a/doc/sphinx/whats-new/upgrading-6.1.rst b/doc/sphinx/whats-new/upgrading-6.1.rst
index 2a33f67e0..73b8dfb72 100644
--- a/doc/sphinx/whats-new/upgrading-6.1.rst
+++ b/doc/sphinx/whats-new/upgrading-6.1.rst
@@ -88,9 +88,14 @@ bit. ``keep`` sets a lifetime in the cache in addition to TTL for
 objects that can be validated by a 304 "Not Modified" response from
 the backend to a conditional request (with ``If-None-Match`` or
 ``If-Modified-Since``). If an expired object is also out of grace
-time, it is no longer possible to deliver a "keep" object from
-``vcl_hit``. It is possible to validate a 304 candidate from
-``vcl_miss``.
+time, then ``vcl_hit`` will no longer be called, so it is impossible
+to deliver the "keep" object in this case.
+
+Note that the headers ``If-None-Match`` and ``If-Modified-Since``,
+together with the 304 behavior, are handled automatically by Varnish.
+If you, for some reason, need to explicitly disable this for a backend
+request, then you need do this by removing the headers in
+``vcl_backend_fetch``.
 
 The documentation in :ref:`users-guide-handling_misbehaving_servers`
 has been expanded to discuss these matters in greater depth, look