Further tweak Carbons text after community discussion

xep-0280.xml

@@ -235,10 +235,12 @@
         <ul>
             <li>A &MESSAGE; is eligible for carbons delivery if it is of type "chat".</li>
             <li>A &MESSAGE; is eligible for carbons delivery if it is of type "normal" and it contains a &lt;body&gt; element.</li>
+            <li>A &MESSAGE; is eligible for carbons delivery if it is of type "error" and sent in response to a &MESSAGE; that was itself eligible for carbons delivery (Note that as this would require message tracking and correlation on the server, it is unlikely to be generally appropriate for most implementations).</li>
             <li>A &MESSAGE; is not eligible for carbons delivery if it is determined to have been sent by a MUC room or service, even if it would be otherwise eligible.</li>
             <li>A &MESSAGE; is not eligible for carbons delivery if it does not meet any of these criteria.</li>
         </ul>
     </p>
+    <p>As this is a implementation detail of servers, clients MUST NOT rely on the server implementing a particular set of rules for which messages are eligible for Carbons delivery.</p>
     <p>Future specifications may have more precise requirements on which messages need to be eligible for carbons delivery; such future specifications will provide their own discovery and negotiation mechanisms, such that a client negotiating Carbons using the protocol defined in this specification will cause the server to consider messages eligible for Carbons delivery based on the requirements described herein.</p>
     <p>Note: previous versions of this specification limited eligible messages to those of type "chat" - however, this was generally found to be inadequate due to the proliferation of type "normal" messages used in instant messaging.</p>
 </section1>