doc: Mention the effect of Connection:close in beresp

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

@@ -38,6 +38,25 @@ Changes to VCL
 VCL variables
 ~~~~~~~~~~~~~
 
+It is now possible to manually set a ``Connection: close`` header in
+``beresp`` to signal that the backend connection shouldn't be recycled.
+This might help dealing with backends that would under certain circumstances
+have trouble managing their end of the connection, for example for certain
+kinds of resources.
+
+Care should be taken to preserve other headers listed in the connection
+header::
+
+    sub vcl_backend_response {
+        if (beresp.backend == faulty_backend) {
+            if (beresp.http.Connection) {
+                set beresp.http.Connection += ", close";
+            } else {
+                set beresp.http.Connection = "close";
+            }
+        }
+    }
+
 **XXX new, deprecated or removed variables, or changed semantics**
 
 Other changes to VCL