Fix #135:  Document / enforce gcloud.datastore.key.Key.parent() contract

See #135
I'm fine for it being immutable but these are not `@property` methods and don't define `__get__` so aren't descriptors.

Thus it's really a method which is somewhat overloaded in it's scope (though I'll acknowledge the code is typically short).

I'd be much more in favor of two separate methods for getting and setting, i.e.
`update_dataset` as a method and `dataset` decorated with `@property` as a getter only.

Moving this to a new issue so the discussion doesn't get lost under the fold.