Add notes explaining how <dialog>.showModal() focuses

It was unclear how the "focus fixup rule" came into play, since that
rule is done using action at a distance. This inserts notes explicitly
calling out what happens.

See web-platform-tests/wpt#23485 for background.

source

@@ -56626,8 +56626,17 @@ interface <dfn>HTMLDialogElement</dfn> : <span>HTMLElement</span> {
 
    <li><p>Set the <code>dialog</code> to the <span>centered alignment</span> mode.</p></li>
 
-   <li><p>Let <var>subject</var>'s <span>node document</span> be <span data-x="blocked by a
-   modal dialog">blocked by the modal dialog</span> <var>subject</var>.</p></li>
+   <li>
+    <p>Let <var>subject</var>'s <span>node document</span> be <span data-x="blocked by a
+    modal dialog">blocked by the modal dialog</span> <var>subject</var>.</p>
+
+    <p class="note" id="note-dialog-plus-focus-fixup">This will cause the <span>focused area of the
+    document</span> to become <span>inert</span> (unless that currently focused area is a
+    <span>shadow-including descendant</span> of <var>subject</var>). In such cases, the <span>focus
+    fixup rule</span> will kick in and reset the <span>focused area of the document</span> to the
+    <span>viewport</span> for now. In a couple steps we will attempt to find a better candidate to
+    focus.</p>
+   </li>
 
    <li><p>If <var>subject</var>'s <span>node document</span>'s <span>top layer</span> does not
    already <span data-x="list contains">contain</span> <var>subject</var>, then <span
@@ -56653,7 +56662,13 @@ interface <dfn>HTMLDialogElement</dfn> : <span>HTMLElement</span> {
     <p>If there isn't one of those either, then let <var>control</var> be <var>subject</var>.</p>
    </li>
 
-   <li><p>Run the <span>focusing steps</span> for <var>control</var>.</p></li>
+   <li>
+    <p>Run the <span>focusing steps</span> for <var>control</var>.</p>
+
+    <p class="note">If <var>control</var> is not <span>focusable</span>, this will do nothing. For
+    modal dialogs, this means that any <a href="#note-dialog-plus-focus-fixup">earlier
+    modifications</a> to the <span>focused area of the document</span> will apply.</p>
+   </li>
 
    <li><p>Let <var>topDocument</var> be the <span>active document</span> of <var>control</var>'s
    <span>node document</span>'s <span data-x="concept-document-bc">browsing context</span>'s