Add missing value for hidden in the index

Closes #8414.

Signed-off-by: Jens Oliver Meiert <[email protected]>

Author: j9t

source

@@ -126130,7 +126130,9 @@ interface <dfn interface>External</dfn> {
      <th> <code data-x="">hidden</code>
      <td> <span data-x="attr-hidden">HTML elements</span>
      <td> Whether the element is relevant
-     <td> <span>Boolean attribute</span>
+     <td> "<code data-x="attr-hidden-until-found-keyword">until-found</code>";
+          "<code data-x="attr-hidden">hidden</code>";
+          the empty string
     <tr>
      <th> <code data-x="">high</code>
      <td> <code data-x="attr-meter-high">meter</code>