[6.0] fc0ca5fb7 improve do_stream documentation

[Dridi Boukelmoune]

commit fc0ca5fb7a1e6c4a6a501a8290e7db11124f4ef4
Author: Nils Goroll <nils.goroll at uplex.de>
Date:   Wed Apr 4 14:56:59 2018 +0200

    improve do_stream documentation

diff --git a/doc/sphinx/reference/vcl_var.rst b/doc/sphinx/reference/vcl_var.rst
index c4ded7c3e..85c11fb22 100644
--- a/doc/sphinx/reference/vcl_var.rst
+++ b/doc/sphinx/reference/vcl_var.rst
@@ -719,13 +719,17 @@ beresp.do_stream
 
 	Writable from: vcl_backend_response, vcl_backend_error
 
+	Default: true
+
 	Deliver the object to the client while fetching the whole
 	object into varnish.
 
 	For uncacheable objects, storage for parts of the body which
 	have been sent to the client may get freed early, depending
 	on the storage engine used.
-	
+
+	This variable has no effect if do_esi is true and when the
+	response body is empty.
 
 beresp.do_gzip