Tooltips for masthead links (#1380)

docs/_docs/07-navigation.md

@@ -34,6 +34,8 @@ Which will give you a responsive masthead similar to this:
 
 ![priority plus masthead animation]({{ "/assets/images/mm-priority-plus-masthead.gif" | absolute_url }})
 
+Optionally, you can add a `description` key per title in the `main` key. This `description` will show up like a tooltip, when the user hovers over the link on a desktop browser.
+
 **ProTip:** Put the most important links first so they're always visible and not hidden behind the **menu toggle**.
 {: .notice--info}