The Role Help module allows a site administrator (with the 'administer access control' permission) to set a summary and help text for each role.
The role help text may be shown to users on a site help page. This works in the same way as the filter tips page: only the role descriptions a user has access to are shown.
The summaries are shown in the role admin page and in forms that show role checkboxes. If you have Role delegation installed, the delegation form shows these too.