Add notes on spec compliance and returning errors

extensions/inbox/rayo.xml

@@ -1650,7 +1650,7 @@
   <p>Further commands and components may also be added to the Rayo protocol in order to extend its capabilities. Such extensions should be submitted to the XSF as ProtoXEPs and use namespaces aligning with the core component namespaces.</p>
 </section1>
 <section1 topic='Implementation Notes' anchor='impl'>
-
+  <p>A server MUST document any cases where its behaviour differs from that in this specification (such as lack of support for particular options/components/etc) and return an error whenever a command is not understood. A server MUST NOT silently ignore any instructions.</p>
 </section1>
 <section1 topic='Security Considerations' anchor='security'>
   <section2 topic='Denial of Service' anchor='security-dos'>