I see the title attribute in use, but of course the user has to hover the button to see it. So, although it's a good idea, it is not obvious. I would combine it with something along the lines of the following.
I am not sure how universal the asterisk (Sternchen) is, so maybe a "yield" or "exclamation triangle" would be more universally understood. But in English, an asterisk signals to the reader that they should look for a reference note, usually at the bottom of the page.