doc: fix util.isObject documentation

Proposed functionality fix containing prior discussion:
https://github.com/iojs/io.js/pull/822

Fixes: https://github.com/iojs/io.js/issues/743
PR-URL: https://github.com/iojs/io.js/pull/1295


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Brendan Ashworth <brendan.ashworth@me.com>