doc: clarify return value of http.createServer
doc: clarify return value of http.createServer

PR-URL: https://github.com/joyent/node/pull/6054
Reviewed-by: Isaac Z. Schlueter <042dc4512fa3d391c5170cf3aa61e6a638f84342@izs.me>
Reviewed-by: Chris Dickinson <38a75c4c4f5646d63b752460df745c5b937b7e58@gmail.com>

dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node