Clarify that <details> toggle events happen during parsing

Closes #4500.
Tests: web-platform-tests/wpt#16244

Also moves the task source inline, per #4506.

source

@@ -56959,9 +56959,9 @@ interface <dfn>HTMLDetailsElement</dfn> : <span>HTMLElement</span> {
   affordance.</p>
 
   <p>Whenever the <code data-x="attr-details-open">open</code> attribute is added to or removed from
-  a <code>details</code> element, the user agent must <span>queue a task</span> that runs the
-  following steps, which are known as the <dfn>details notification task steps</dfn>, for this
-  <code>details</code> element:</p>
+  a <code>details</code> element, the user agent must <span>queue a task</span> on the <span>DOM
+  manipulation task source</span> to run the following steps, which are known as the <dfn>details
+  notification task steps</dfn>, for this <code>details</code> element:</p>
 
   <ol>
 
@@ -56982,8 +56982,7 @@ interface <dfn>HTMLDetailsElement</dfn> : <span>HTMLElement</span> {
 
   </ol>
 
-  <p>The <span>task source</span> for this task must be the <span>DOM manipulation task
-  source</span>.</p>
+  <p class="note">To be clear, this also happens when the attribute is added via the parser.</p>
 
   <p>The <dfn><code data-x="dom-details-open">open</code></dfn> IDL attribute must
   <span>reflect</span> the <code data-x="attr-details-open">open</code> content attribute.</p>