[wwwdocs] Document defining _GLIBCXX_ASSERTIONS for -O0

[Jonathan Wakely]

---
Pushed to wwwdocs.

 htdocs/gcc-15/changes.html | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/htdocs/gcc-15/changes.html b/htdocs/gcc-15/changes.html
index ad0daf04..805a6703 100644
--- a/htdocs/gcc-15/changes.html
+++ b/htdocs/gcc-15/changes.html
@@ -93,6 +93,9 @@ a work-in-progress.</p>
 <h4 id="libstdcxx">Runtime Library (libstdc++)</h4>
 
 <ul>
+  <li>Debug assertions are now enabled by default for unoptimized builds.
+    Use <code>-D_GLIBCXX_NO_ASSERTIONS</code> to override this.
+  </li>
   <li>Improved experimental support for C++26, including:
     <ul>
     <li><code>views::concat</code>.</li>
-- 
2.46.2