doc: document http's server.listen return value

The server method returns `self` in order to allow chaining.

PR-URL: https://github.com/nodejs/node/pull/4590
Fixes: https://github.com/nodejs/node/issues/4571
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>

doc/api/http.markdown

@@ -506,6 +506,8 @@ Listening on a file descriptor is not supported on Windows.
 This function is asynchronous. The last parameter `callback` will be added as
 a listener for the `'listening'` event. See also [`net.Server.listen()`][].
 
+Returns `server`.
+
 ### server.listen(path[, callback])
 
 Start a UNIX socket server listening for connections on the given `path`.