Editorial: use relevant agent for agent cluster definitions

This also addresses a missing agent variable introduction.

source

@@ -88952,17 +88952,16 @@ import "https://example.com/foo/../module2.mjs";</code></pre>
   worker agent</span>, or <span>service worker agent</span>, <var>agent</var>, <span>can share
   memory with</span> any <span>dedicated worker agent</span> whose single <span data-x="JavaScript
   realm">realm</span>'s <span data-x="concept-realm-global">global object</span>'s <span>owner
-  set</span> contains an item whose <span data-x="concept-relevant-realm">relevant Realm</span>
-  belongs to <var>agent</var>.</p>
+  set</span> contains an item whose <span>relevant agent</span> is <var>agent</var>.</p>
 
   <p class="note">"Item" is used above as an <span>owner set</span> can contain
   <code>Document</code> objects.</p>
 
-  <p>A <span>similar-origin window agent</span> <span>can share memory with</span> any
-  <span>worklet agent</span> whose single <span data-x="JavaScript realm">realm</span>'s
+  <p>A <span>similar-origin window agent</span> <var>agent</var> <span>can share memory with</span>
+  any <span>worklet agent</span> whose single <span data-x="JavaScript realm">realm</span>'s
   <span data-x="concept-realm-global">global object</span>'s
-  <span data-x="concept-worklet-owner-document">owner document</span>'s
-  <span data-x="concept-relevant-realm">relevant Realm</span> belongs to <var>agent</var>.</p>
+  <span data-x="concept-worklet-owner-document">owner document</span>'s <span>relevant agent</span>
+  is <var>agent</var>.</p>
 
   <p>In addition, any <span>agent</span> <var>A</var> <span>can share memory with</span>:</p>