Fix case where the directionality of a <bdi> could be null.

In whatwg#9796 I refactored the term "auto directionality" to move the
fallback to the parent directionality out of the definition of "auto
directionality", since I needed the null result in one case to continue
a tree traversal rather than returning.

However, I missed fixing up one of the references to the term (should I
blame the line break?), leaving a case where the spec could define the
directionality of a <bdi> element as null.

This fixes that case, and converts the <ol> to a <dl class="switch">.

source

@@ -13599,19 +13599,24 @@ Transport Protocol">HTTP&lt;/abbr> today.&lt;/p></code></pre> <!-- DO NOT REWRAP
 
    <dt><span data-x="attr-dir-undefined-state">undefined</span></dt>
    <dd>
-    <ol>
-     <li><p>If <var>element</var> is a <code>bdi</code> element, then return the <span>auto
-     directionality</span> of <var>element</var>.</p></li>
+    <dl class="switch">
+     <dt>If <var>element</var> is a <code>bdi</code> element and the <span>auto
+     directionality</span> of <var>element</var> is not null</dt>
 
-     <li><p>If <var>element</var> is an <code>input</code> element whose <code
+     <dd><p>Return the <span>auto directionality</span> of <var>element</var>.</p></dd>
+
+     <dt>If <var>element</var> is an <code>input</code> element whose <code
      data-x="attr-input-type">type</code> attribute is in the <span
-     data-x="attr-input-type-tel">Telephone</span> state, then return '<span
-     data-x="concept-ltr">ltr</span>'.</p></li>
+     data-x="attr-input-type-tel">Telephone</span> state</dt>
+
+     <dd><p>Return '<span data-x="concept-ltr">ltr</span>'.</p></dd>
      <!-- This looks odd, but is very much deliberate:
           https://www.w3.org/Bugs/Public/show_bug.cgi?id=17835 -->
 
-     <li><p>Return the <span>parent directionality</span> of <var>element</var>.</p></li>
-    </ol>
+     <dt>Otherwise</dt>
+
+     <dd><p>Return the <span>parent directionality</span> of <var>element</var>.</p></dd>
+    </dl>
    </dd>
   </dl>