[wwwdocs] Document libstdc++ changes in gcc-12

[Jonathan Wakely]

Pushed to wwwdocs.

---
 htdocs/gcc-12/changes.html | 21 ++++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

diff --git a/htdocs/gcc-12/changes.html b/htdocs/gcc-12/changes.html
index 19947f90..5f12fb42 100644
--- a/htdocs/gcc-12/changes.html
+++ b/htdocs/gcc-12/changes.html
@@ -165,7 +165,26 @@ a work-in-progress.</p>
   </li>
 </ul>
 
-<!-- <h4 id="libstdcxx">Runtime Library (libstdc++)</h4> -->
+<h4 id="libstdcxx">Runtime Library (libstdc++)</h4>
+
+<ul>
+<li>Improved experimental C++20 support, including:
+  <ul>
+  <li><code>std::vector</code>, <code>std::optional</code>,
+      and <code>std::variant</code> can be used in <code>constexpr</code>
+      functions.</li>
+  <li>Layout-compatibility and pointer-interconvertibility traits.</li>
+  </ul>
+</li>
+<li>Improved experimental C++23 support, including:
+  <ul>
+  <li>Monadic operations for <code>std::optional</code>.</li>
+  <li><code>std::move_only_function</code></li>
+  <li><code>std::invoke_r</code></li>
+  <li><code>std::basic_string::resize_and_overwrite</code></li>
+  </ul>
+</li>
+</ul>
 
 <!-- <h3 id="d">D</h3> -->
 
-- 
2.31.1