Clarify that synchronous functions in fs with no return value return
undefined.
Specify that fs.openSync() returns an integer and fs.existsSync()
returns true or false.
Fixes#9313
PR: #9359
PR-URL: https://github.com/joyent/node/pull/9359
Reviewed-By: Julien Gilli <julien.gilli@joyent.com>