Re-add snippet needed for websockets support

Pointed out by nirn on irc.

doc/sphinx/users-guide/vcl-example-websockets.rst

@@ -5,10 +5,17 @@ Adding WebSockets support
 WebSockets is a technology for creating a bidirectional stream-based
 channel over HTTP.
 
-To run WebSockets through Varnish you need to pipe it as follows::
+To run WebSockets through Varnish you need to pipe the request and copy
+the Upgrade header as follows::
 
     sub vcl_recv {
         if (req.http.upgrade ~ "(?i)websocket") {
             return (pipe);
         }
     }
+
+    sub vcl_pipe {
+        if (req.http.upgrade) {
+            set bereq.http.upgrade = req.http.upgrade;
+        }
+    }