[wwwdocs] Document synchronized_value addition to libstdc++

[Jonathan Wakely]

Pushed to wwwdocs.


---
 htdocs/gcc-13/changes.html | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/htdocs/gcc-13/changes.html b/htdocs/gcc-13/changes.html
index a803f501..811d9bdf 100644
--- a/htdocs/gcc-13/changes.html
+++ b/htdocs/gcc-13/changes.html
@@ -324,6 +324,9 @@ a work-in-progress.</p>
   <li>Support for the <code><experimental/scope></code> header
       from v3 of the Library Fundamentals Technical Specification.
   </li>
+  <li>Support for the <code><experimental/synchronized_value></code>
+      header from v2 of the Concurrency Technical Specification.
+  </li>
 </ul>
 
 <!-- <h3 id="d">D</h3> -->
-- 
2.39.2