Assuming you are a wiki administrator and ready to display it on the mouse, not on the click, you don't need javascript at all - you can use direct CSS. Here is an example of styles and layouts:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<title>Test</title>
<style type="text/css">
h1 { padding-bottom: .5em; position: relative; }
h1 span { font-weight: normal; font-size: small; position: absolute; bottom: 0; display: none; }
h1:hover span { display: block; }
</style>
</head>
<body>
<h1>Here is the title!
<span>Here is a little explanation</span>
</h1>
<p>Here is some page content</p>
</body>
</html>
With a few more styles involved, your toolbar may look as good as we would like.