Add note from requestIdleCallback spec

source

@@ -93551,8 +93551,13 @@ import "https://example.com/foo/../module2.mjs";</code></pre>
     following steps to compute the current idle period deadline: <ref spec=REQUESTIDLECALLBACK></p>
 
     <ol>
-     <li><p>Let <var>deadline</var> be this <span>event loop</span>'s
-     <span>last idle period start time</span> plus 50.</p></li>
+     <li>
+      <p>Let <var>deadline</var> be this <span>event loop</span>'s
+      <span>last idle period start time</span> plus 50.</p>
+
+      <p class="note">The cap of 50ms in the future is to ensure responsiveness to new user input
+      within the threshold of human perception.</p>
+     </li>
 
      <li><p>Check for potential pending rendering: If any <code>Window</code> in the
      <span>same-loop windows</span> for this <span>event loop</span>'s <span>map of animation frame