Clarify support for multiple headers of the same name

Fixes #35

extensions/inbox/rayo.xml

@@ -1817,6 +1817,24 @@
       ]]></example>
     </section3>
   </section2>
+
+  <section2 topic='Headers' anchor='session-headers'>
+    <p>In elements which may carry child &lt;header/&gt; elements, a server or client MAY specify several header elements with the same name. In such cases, these MUST be considered to form a collection of ordered values for the key provided.</p>
+
+    <example caption="Client requests establishment of a new outbound session with multiple values for the SIP Route header"><![CDATA[
+<iq from='[email protected]/balcony'
+    to='shakespeare.lit'
+    type='set'
+    id='h7ed2'>
+  <dial xmlns='urn:xmpp:rayo:1'
+        to='tel:+13055195825'
+        from='tel:+14152226789'>
+    <header name="Route" value="foo" />
+    <header name="Route" value="bar" />
+  </dial>
+</iq>
+    ]]></example>
+  </section2>
 </section1>
 <section1 topic='Formal Definition' anchor='def'>