Add "relevant agent" and point out "surrounding agent"

These would improve whatwg/dom#717 and #4204.

source

@@ -3001,6 +3001,7 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
      <li><dfn data-x-href="https://tc39.github.io/ecma262/#execution-context-stack">JavaScript execution context stack</dfn></li>
      <li><dfn data-x-href="https://tc39.github.io/ecma262/#sec-code-realms">JavaScript realm</dfn></li>
      <li><dfn data-x-href="https://tc39.github.io/ecma262/#running-execution-context">running JavaScript execution context</dfn></li>
+     <li><dfn data-x-href="https://tc39.github.io/ecma262/#surrounding-agent">surrounding agent</dfn></li>
      <li><dfn data-x-href="https://tc39.github.io/ecma262/#use-strict-directive">Use Strict Directive</dfn></li>
      <li><dfn data-x-href="https://tc39.github.io/ecma262/#sec-well-known-symbols">Well-Known Symbols</dfn>, including
          <dfn>@@hasInstance</dfn>,
@@ -88917,6 +88918,16 @@ import "https://example.com/foo/../module2.mjs";</code></pre>
    </dd>
   </dl>
 
+  <hr>
+
+  <p>The <dfn data-export="">relevant agent</dfn> for a <span>platform object</span>
+  <var>platformObject</var> is the <span>agent</span> whose set of <span data-x="JavaScript
+  realm">realms</span> contains <var>platformObject</var>'s <span
+  data-x="concept-relevant-realm">relevant Realm</span>.</p>
+
+  <p class="note">The agent equivalent of the <span>current Realm Record</span> is the
+  <span>surrounding agent</span>.</p>
+
   <h5>Integration with the JavaScript agent cluster formalism</h5>
 
   <p>JavaScript also defines the concept of an <span>agent cluster</span>, which this standard maps