Rendering: further details/summary cleanup

As is evident from https://bugs.chromium.org/p/chromium/issues/detail?id=973074 the current description still had too much from the old description and led to some inconsistencies in implementations. This makes it clear the slots have no special styling (by default) and it's the first summary element that is special, not the first slot.

Tests: web-platform-tests/wpt#26053.

source

@@ -114889,16 +114889,16 @@ details[open] > summary {
 
   <p>The <code>details</code> element is expected to render as a <span>block box</span>. The element
   is also expected to have an internal <span>shadow tree</span> with two <span
-  data-x="concept-slot">slots</span>, both rendered as a <span>block box</span>. The first <span
-  data-x="concept-slot">slot</span> is expected to take the <code>details</code> element's first
-  <code>summary</code> element child, if any. The second <span data-x="concept-slot">slot</span> is
-  expected to take the <code>details</code> element's remaining descendants, if any.</p>
+  data-x="concept-slot">slots</span>. The first <span data-x="concept-slot">slot</span> is expected
+  to take the <code>details</code> element's first <code>summary</code> element child, if any. The
+  second <span data-x="concept-slot">slot</span> is expected to take the <code>details</code>
+  element's remaining descendants, if any.</p>
 
-  <p>The first <span data-x="concept-slot">slot</span> is expected to allow the user to request the
-  details be shown or hidden.</p>
+  <p>The <code>details</code> element's first <code>summary</code> element child, if any, is
+  expected to allow the user to request the details be shown or hidden.</p>
 
-  <p>The second <span data-x="concept-slot">slot</span> is expected to be removed from the rendering
-  when the <code>details</code> element does not have an <code
+  <p>The <code>details</code> element's second <span data-x="concept-slot">slot</span> is expected
+  to be removed from the rendering when the <code>details</code> element does not have an <code
   data-x="attr-details-open">open</code> attribute.</p>
 
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