doc: document callback of http res/req end method

Add documentation for the callback parameter of http.ClientRequest's and
http.ServerResponse's end methods.

PR-URL: https://github.com/iojs/io.js/pull/181


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Brendan Ashworth <squirrelslikeacorns@gmail.com>