[5.2] 1c55d49 Mention the addition of bereq.is_bgfetch in Upgrading to 5.2.

[PÃ¥l Hermunn Johansen]

commit 1c55d4991ff3e4d2ab7980d77c05ac49f5f4d27c
Author: Geoff Simmons <geoff at uplex.de>
Date:   Wed Sep 13 17:49:19 2017 +0200

    Mention the addition of bereq.is_bgfetch in Upgrading to 5.2.
    
    Since we seem to have a number of new VCL variables, there is now
    a section heading for that in Upgrading.

diff --git a/doc/sphinx/whats-new/upgrading-5.2.rst b/doc/sphinx/whats-new/upgrading-5.2.rst
index 00abba5..ee281eb 100644
--- a/doc/sphinx/whats-new/upgrading-5.2.rst
+++ b/doc/sphinx/whats-new/upgrading-5.2.rst
@@ -72,8 +72,11 @@ dashes in a vmod symbol.
 Long storage backend names used to be truncated due to a limitation in
 the VSC subsystem, this is no longer the case.
 
+New VCL variables
+~~~~~~~~~~~~~~~~~
+
 ``req.hash`` and ``bereq.hash``
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+-------------------------------
 
 Added ``req.hash`` and ``bereq.hash``, which contain the hash value
 computed by Varnish for cache lookup in the current transaction, to
@@ -95,7 +98,7 @@ You can use :ref:`vmod_blob(3)` to work with the hashes::
   }
 
 ``server.identity``
-~~~~~~~~~~~~~~~~~~~
+-------------------
 
 If the ``-i`` option is not set in the invocation of ``varnishd``,
 then ``server.identity`` is set to the host name (as returned by
@@ -103,6 +106,16 @@ then ``server.identity`` is set to the host name (as returned by
 value of the ``-n`` option (or the default instance name if ``-n`` was
 not set). See :ref:`varnishd(1)`.
 
+``bereq.is_bgfetch``
+--------------------
+
+``bereq.is_bgfetch`` is readable in backend contexts, and is true if
+the fetch takes place in the background. That is, it is true if
+Varnish found a response in the cache whose TTL was expired, but was
+still in grace time. Varnish returns the stale cached response to the
+client, and initiates the background fetch to refresh the cache
+object.
+
 XXX: vcl_sub_XXX ...
 ~~~~~~~~~~~~~~~~~~~~