By self-referential, we mean that it talks about itself. By self-documenting we mean that it makes a (true) statement about its own contents. For example, it may make a true statement about the number of occurrences of each of the digits 0 to 9 that it contains.
It is very important that the statement it makes is true, because there are plenty of statements of this kind that are false. Just try putting a few randomly chosen numbers into the sentence instead: it is highly unlikely that you will get a true statement just by chance!