Editorial: clarify the difference between :stalled pseudo-class and stalled event

See #12145 (comment) for context.

Author: alastor0325

source

@@ -79704,10 +79704,17 @@ Demos:
    </dd>
 
    <dt><dfn selector noexport><code data-x="selector-stalled">:stalled</code></dfn></dt>
-   <dd><p>The <code data-x="selector-stalled">:stalled</code> <span>pseudo-class</span> must match
-   all <span data-x="media element">media elements</span> that match the <code
-   data-x="selector-buffering">:buffering</code> <span>pseudo-class</span> and whose <span>is
-   currently stalled</span> is true.</p></dd>
+   <dd>
+    <p>The <code data-x="selector-stalled">:stalled</code> <span>pseudo-class</span> must match
+    all <span data-x="media element">media elements</span> that match the <code
+    data-x="selector-buffering">:buffering</code> <span>pseudo-class</span> and whose <span>is
+    currently stalled</span> is true.</p>
+
+    <p class="note">This pseudo-class is intended for displaying player state UI when video playback
+    has been buffering for too long. Unlike the <code
+    data-x="event-media-stalled">stalled</code> event, which fires whenever the network download has
+    stalled regardless of playback or ready state.</p>
+   </dd>
 
    <dt><dfn selector noexport><code data-x="selector-muted">:muted</code></dfn></dt>
    <dd><p>The <code data-x="selector-muted">:muted</code> <span>pseudo-class</span> must match all