[wwwdocs] Add some C++23 and C++26 library features to GCC 16 release notes

[Jonathan Wakely]

Also thank Tomasz for std::format range support in GCC 15
---

Pushed to wwwdocs.

 htdocs/gcc-15/changes.html |  2 +-
 htdocs/gcc-16/changes.html | 21 ++++++++++++++++++++-
 2 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/htdocs/gcc-15/changes.html b/htdocs/gcc-15/changes.html
index bf980491..23632d41 100644
--- a/htdocs/gcc-15/changes.html
+++ b/htdocs/gcc-15/changes.html
@@ -679,7 +679,7 @@ asm (".text; %cc0: mov %cc2, %%r0; .previous;"
     </li>
     <li>
       Formatting of ranges and tuples with <code>std::format</code>,
-      as well as string escaping for debug formats.
+      as well as string escaping for debug formats, thanks to Tomasz Kamiński.
     </li>
     <li>
       Clarify handling of encodings in localized formatting of chrono types.
diff --git a/htdocs/gcc-16/changes.html b/htdocs/gcc-16/changes.html
index 99644758..cc6fe204 100644
--- a/htdocs/gcc-16/changes.html
+++ b/htdocs/gcc-16/changes.html
@@ -75,7 +75,26 @@ for general information.</p>
 
 <h3 id="cxx">C++</h3>
 
-<!-- <h4 id="libstdcxx">Runtime Library (libstdc++)</h4> -->
+<h4 id="libstdcxx">Runtime Library (libstdc++)</h4>
+
+<ul>
+  <li>Improved experimental support for C++26, including:
+    <ul>
+    <li>
+      <code>std::copyable_function</code> and <code>std::function_ref</code>.
+    </li>
+    <li><code>std::indirect</code> and <code>std::polymorphic</code>.</li>
+    <li>
+      <code>std::owner_equal</code> for shared pointers, thanks to Paul Keir.
+    </li>
+    </ul>
+  </li>
+  <li>Improved experimental support for C++23, including:
+    <ul>
+    <li><code>std::mdspan</code>, thanks to Luc Grosheintz.</li>
+    </ul>
+  </li>
+</ul>
 
 <!-- <h3 id="d">D</h3> -->
 
-- 
2.50.0