Modal docs: Add MDN autofocus docs link

docs/components/modal.md

@@ -11,7 +11,7 @@ Modals are streamlined, but flexible, dialog prompts with the minimum required f
 * Will be replaced with the ToC, excluding the "Contents" header
 {:toc}
 
-**Due to how HTML5 defines its semantics, the `autofocus` HTML attribute has no effect in Bootstrap modals.** To achieve the same effect, use some custom JavaScript:
+**Due to how HTML5 defines its semantics, [the `autofocus` HTML attribute](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/input#attr-autofocus) has no effect in Bootstrap modals.** To achieve the same effect, use some custom JavaScript:
 
 {% highlight js %}
 $('#myModal').on('shown.bs.modal', function () {