Remove "atrisk" note on HTML document base

This was resolved in w3c/json-ld-syntax#134 (comment) which is reflected in the syntax document.

Author: gkellogg

index.html

@@ -5886,11 +5886,6 @@ <h2>HTML Content Algorithms</h2>
         <li>Set <var>documentUrl</var> to the the <a data-cite="HTML/urls-and-fetching.html#document-base-url">Document Base URL</a>
           of <a data-link-for="LoadDocumentCallback">url</a>, as defined in [[HTML]],
           using the existing <var>documentUrl</var> as the document's URL.
-          <div class="issue atrisk">
-            The use of the <a data-cite="HTML/urls-and-fetching.html#document-base-url">Document Base URL</a>
-            from [[HTML]] for setting the <a>base IRI</a> of the enclosed JSON-LD
-            is an experimental feature, which may be changed in a future version of this specification.
-          </div>
         </li>
         <li>If the <a data-link-for="LoadDocumentCallback">url</a> parameter
           contains a <a data-cite="RFC3986#section-3.5">fragment identifier</a>,