Correct doc block

When the input is an empty string the doc block is wrong and misleading.
It will return the string again - I would have expected an empty array, though.

I would prefer to drop the is_array() check (nonsense since we expect a string) and return empty array() if empty:

	if (empty($data)) {
		return array();
	}

But that will have to wait for 2.5 then.