docs: add return value for sync fs functions

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>