docs: add return value for sync fs functions

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: https://github.com/joyent/node/issues/9313

PR: https://github.com/joyent/node/pull/9359
Reviewed-By: Julien Gilli <julien.gilli@joyent.com>
PORT-FROM: joyent/node @ 51fe319faf4399fd027f8b32d1c425200b911e44
PR-URL: https://github.com/nodejs/io.js/pull/1770
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Conflicts:
	doc/api/fs.markdown