add comment regarding aria-details

see https://bugs.chromium.org/p/chromium/issues/detail?id=1426613#c4

aria-details with an empty string could be useful to authors if there was a specific need to _not_ expose a child figcaption - e.g., if the elements need to have remapped roles for whatever reason.

index.html

@@ -1040,6 +1040,9 @@ <h3>HTML Element Role Mappings</h3>
                 <p>If an author specifies a `figure` element to acquire its accessible name or description from the first instance
                   of a `figcaption` with an `aria-labelledby` or `aria-describedby` association, then user agents SHOULD NOT expose a 
                   details relationship between the `figure` and `figcaption` elements.</p>
+                <p>An explicit declaration of the `aria-details` attribute on the `figure` element will override the implicit `aria-details` 
+                   association with the element's child `figcaption` element, if one exists. If the `aria-details` attribute has an empty value,
+                  then the `figure` element has no details relation to expose.</p>
               </td>
             </tr>
             <tr tabindex="-1" id="el-figure">