Editorial: cross-reference the "exposed" IDL concept

Fixes whatwg/webidl#518.

source

@@ -2920,6 +2920,7 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
      <li><dfn data-x-href="https://heycam.github.io/webidl/#createsyntheticmodule">CreateSyntheticModule</dfn></li>
      <li><dfn data-x-href="https://heycam.github.io/webidl/#setsyntheticmoduleexport">SetSyntheticModuleExport</dfn></li>
      <li><dfn data-x-href="https://heycam.github.io/webidl/#synthetic-module-record">Synthetic Module Record</dfn></li>
+     <li><dfn data-x="idl-exposed" data-x-href="https://heycam.github.io/webidl/#dfn-exposed">exposed</dfn></li>
     </ul>
 
     <p>The <cite>Web IDL</cite> also defines the following types that are used in Web IDL fragments
@@ -8885,9 +8886,9 @@ o.myself = o;</code></pre>
     <ol>
      <li><p>Let <var>interfaceName</var> be <var>serialized</var>.[[Type]].</p></li>
 
-     <li><p>If the interface identified by <var>interfaceName</var> is not exposed in
-     <var>targetRealm</var>, then throw a <span>"<code>DataCloneError</code>"</span>
-     <code>DOMException</code>.</p></li>
+     <li><p>If the interface identified by <var>interfaceName</var> is not
+     <span data-x="idl-exposed">exposed</span> in <var>targetRealm</var>, then throw a
+     <span>"<code>DataCloneError</code>"</span> <code>DOMException</code>.</p></li>
 
      <li><p>Set <var>value</var> to a new instance of the interface identified by
      <var>interfaceName</var>, created in <var>targetRealm</var>.</p>