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>