The screen tip is intended as a "short" description - therefore not needing
a second line. I don't know if it can be done, or if it automatically CR's
if the line is too long.
There are other ways of adding "screen tips" with little popup windows using
javascript. (involves layers I believe) and setting the property "visible"
to 'On' and 'Off' depending on if you rollover the link with the mouse.