Move an item about the 2018-08 release from 'Changes' to 'Upgrading'.

We have settled on using 'Upgrading' to list changes like this, while
'Changes' is a free-form prose overview ("about this release").

doc/sphinx/whats-new/changes-6.1.rst

@@ -10,8 +10,4 @@ Changes in Varnish 6.1
 
 **XXX**
 
-Varnish now won't rewrite the content-length header when responding to any HEAD
-request, making it possible to cache HEAD requests independently from the GET
-ones (peviously a HEAD request had to be a pass to avoid this rewriting).
-
 *eof*

doc/sphinx/whats-new/upgrading-6.1.rst

@@ -117,6 +117,11 @@ The ``Host`` header is mandatory for HTTP/1.1, as proscribed by the
 HTTP standard. If it is missing, then ``builtin.vcl`` causes a
 synthetic 400 "Bad request" response to be returned.
 
+Varnish now won't rewrite the content-length header when responding to
+any HEAD request, making it possible to cache responses to HEAD
+requests independently from the GET responses (previously a HEAD
+request had to be a pass to avoid this rewriting).
+
 You can now provide a string argument to ``return(fail("Foo!"))``,
 which can be used in ``vcl_init`` to emit an error message if the VCL
 load fails due to the return.