improve do_stream documentation

doc/sphinx/reference/vcl_var.rst

@@ -719,6 +719,8 @@ 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.
 
@@ -726,6 +728,8 @@ beresp.do_stream
 	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