Add a note explaining relList.supports() for feature detection

Closes w3c/preload#126.

Author: noamr

source

@@ -14080,6 +14080,10 @@ interface <dfn interface>HTMLLinkElement</dfn> : <span>HTMLElement</span> {
   IDL attribute must <span data-x="reflect">reflect</span> the <code
   data-x="attr-link-rel">rel</code> content attribute.</p>
 
+  <p class="note">The <code data-x="dom-link-rellist">relList</code> attribute can be used for
+  feature detection, by calling its <code data-x="dom-DOMTokenList-supports">supports()</code>
+  method to check which <a href="#linkTypes">types of links</a> are supported.</p>
+
   <h5>Processing the <code data-x="attr-link-media">media</code> attribute</h5>
 
   <p>If the link is a <span>hyperlink</span> then the <code data-x="attr-link-media">media</code>