This function sets up the little admin help popup boxes.

• Uses the Help template and popup subtemplate
• If this is a "permissionhelp", it loads the ManagePermissions language
• Sets up templates for admin help popup
• Help text is $helptxt[$_GET['help']] (if it exists), $txt[$_GET['help']] (if $helptxt[$_GET['help']] doesn't exist) or just $_GET['help'] if no language string can be found.