doc: clarify return value of http.createServer

PR-URL: https://github.com/joyent/node/pull/6054
Reviewed-by: Isaac Z. Schlueter <i@izs.me>
Reviewed-by: Chris Dickinson <christopher.s.dickinson@gmail.com>

doc/api/http.markdown

@@ -59,7 +59,7 @@ Found'`.
 
 ## http.createServer([requestListener])
 
-Returns a new web server object.
+Returns a new instance of [http.Server](#http_class_http_server).
 
 The `requestListener` is a function which is automatically
 added to the `'request'` event.