Hello,
For this you may use jQuery UI. I managed to do what you want by editing the transformation that displays your document type.
First add this to the particular transformation:
<link href="http://code.jquery.com/ui/1.10.0/themes/base/jquery-ui.css" rel="stylesheet" />
<script src="http://code.jquery.com/jquery-1.8.3.js"></script>
<script src="http://code.jquery.com/ui/1.10.0/jquery-ui.js"></script><script>
  $(function() {
    $( document ).tooltip();
  });
  </script>
<style type="text/css">
</style>
Also use the jQuery always in NoConflict mode as described at 
http://api.jquery.com/jQuery.noConflict/This will register basic jQuery with UI and basic theme. At this point all you need to do is when you are writing some text in the document type you need to switch the 
Source button to begin editing through source code and then you may do as this example:
This is just some <span title="my own tooltip">useless</span> text. 
Now when user hovers over 
useless the tool tip will be shown to him.
You may find more information at 
http://jqueryui.com/tooltip/Hope this helps.
Best regards,
Richard Sustek