Name custom elements in the Extensibility section

Closes #9082.

Author: bkardell

Diff for: source

@@ -420,6 +420,11 @@
    attributes) to embed nested name-value pairs of data to be shared with other applications and
    sites.</p></li>
 
+   <li><p>Authors can define, share, and use <span data-x="custom element">custom elements</span>
+   to extend the vocabulary of HTML. The requirements of <span data-x="valid custom element
+   name">valid custom element names</span> ensure forward compatibility (since no elements will be
+   added to HTML, SVG, or MathML with hyphen-containing local names in the future).</p></li>
+
   </ul>