Warn that the XML syntax is not recommended

Closes #10237.

source

@@ -131812,6 +131812,13 @@ console.assert(container.firstChild instanceof SuperP);
   <code>text/html</code> resources are discussed in the section above entitled "<span>The HTML
   syntax</span>".</p>
 
+  <p class="warning" id="xml-syntax-not-recommended">Using the XML syntax is not recommended, for
+  reasons which include the fact that there is no specification which defines the rules for how an
+  XML parser must map a string of bytes or characters into a <code>Document</code> object, as well
+  as the fact that the XML syntax is essentially unmaintained — in that, it’s not expected that any
+  further features will ever be added to the XML syntax (even when such features have been added to
+  the <span data-x="the HTML syntax">HTML syntax</span>).</p>
+
   <div w-nodev>
 
   <h3 id="writing-xhtml-documents">Writing documents in the XML syntax</h3>