[accname] Explicitly state UAs must ignore “aria-label” for slots

This change updates the AriaLabel substep of the “Computation steps”
section of the “Accessible Name and Description Computation” spec to
explicitly state that the step must not be performed if the node being
visited is a slot element.

accname/index.html

@@ -578,7 +578,7 @@ <h4>Computation steps</h4>
                   </aside>
                 </li>
                 <li id="comp_label">
-                  <span id="step2D"><!-- Don't link to this legacy numbered ID. --></span><em>AriaLabel:</em> Otherwise, if the <code>current node</code> has an <code>aria-label</code> [=attribute=]
+                  <span id="step2D"><!-- Don't link to this legacy numbered ID. --></span><em>AriaLabel:</em> Otherwise, if the <code>current node</code> is not a [=slot=] element and has an <code>aria-label</code> [=attribute=]
                   whose value is not undefined, not the empty string, nor, when trimmed of [=ascii whitespace|whitespace=], is not the empty string:
                   <ol>
                     <li>