XEP-0004: Add note about compatibility for element ordering

After some discussion on-list [1] and in the council [2], we
agreed that an informational box would be best to convey the
change.

   [1]: https://mail.jabber.org/pipermail/standards/2021-February/038148.html
   [2]: https://logs.xmpp.org/council/2021-02-17#2021-02-17-f706cab427273ca0

xep-0004.xml

@@ -295,8 +295,10 @@
       <li>One and only &lt;reported/&gt; element, which can be understood as a "table header" describing the data to follow.</li>
       <li>Zero or more &lt;item/&gt; elements, which can be understood as "table cells" containing data (if any) that matches the request.</li>
     </ol>
-	<p>The &lt;reported/&gt; element MUST appear before any
-	&lt;item/&gt; element in the XML document.</p>
+	<p>The &lt;reported/&gt; element MUST appear before any &lt;item/&gt; element inside the &lt;x/&gt; element.</p>
+  <p class="box info">
+    Older revisions of this XEP (before 2.12.0) did not contain an explicit requirement for the ordering between &lt;reported&gt; and &lt;item&gt;. Implementations are therefore encouraged to be flexible when processing incoming data, as there might still be implementations which do not implement a strict ordering when generating reports.
+  </p>
     <p>The syntax is as follows:</p>
     <code><![CDATA[
 <x xmlns='jabber:x:data'