Clicking, the default assumed action of the override permissions screen, just takes the user to a generic page which does not give an idea of the specific risk in question for a specific override. Since the cursor indicates the target is clickable, users do not wait/expect a tooltip to popup. Make the specific risk description available onclick, with another link to more information in the help that pops up.
Example path where this appears: /moodle2cvs/admin/roles/override.php?contextid=33&roleid=5
Nielsen's heuristics violated: Consistency and standards, Error prevention
Before http://www.pilpi.net/software/moodle/2009/06/18/inline-help/ is implemented as an alternate help solution, a standard moodle help popup is probably the way to go.