[wwwdocs] Add some libstdc++ additions to GCC 15 release notes

[Jonathan Wakely]

Pushed to wwwdocs.

---
 htdocs/gcc-15/changes.html | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/htdocs/gcc-15/changes.html b/htdocs/gcc-15/changes.html
index 18484915..02f8752f 100644
--- a/htdocs/gcc-15/changes.html
+++ b/htdocs/gcc-15/changes.html
@@ -370,6 +370,13 @@ asm (".text; %cc0: mov %cc2, %%r0; .previous;"
   </li>
   <li>Improved experimental support for C++23, including:
     <ul>
+    <li>
+      <code>std</code> and <code>std.compat</code> modules
+      (also supported for C++20).
+    </li>
+    <li>
+      <code>std::flat_map</code> and <code>std::flat_set</code>.
+    </li>
     <li>
       Clarify handling of encodings in localized formatting of chrono types.
     </li>
-- 
2.48.1