util: document the undocumented isXYZ functions

@bnoordhuis's [comment](https://github.com/joyent/node/pull/6341#issuecomment-26245786) implies that they aren't meant to be used outside of node core, but I don't see why – they are useful and have well-defined behavior.

Also, currently `util.isObject` returns `false` for functions, which are full-fledged objects. It could be easily fixed by changing the check to `Object(arg) === arg`. If there is a specific reason why functions need to be excluded, then I'll change the docs to mention it.