Most menu items in Drupal 7 show a link tool-tip, or title, but local tasks (the tabs you are most used to seeing as the edit link tab on full node pages sadly do not follow this sensible pattern.
Until Drupal core is fixed so that menu item "description" is applied to title attribute of menu tab links", here is a fix for Drupal 7. Change this to your custom theme's name (replace 'example') and drop into your theme's template.php file:
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