[wwwdocs, pushed] Document libstdc++ headers that are deprecated in GCC 15

[Jonathan Wakely]

Pushed to wwwdocs.

---
 htdocs/gcc-15/porting_to.html | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/htdocs/gcc-15/porting_to.html b/htdocs/gcc-15/porting_to.html
index c446e309..2bc0d4e5 100644
--- a/htdocs/gcc-15/porting_to.html
+++ b/htdocs/gcc-15/porting_to.html
@@ -155,6 +155,17 @@ may need to be included explicitly when compiling with GCC 15:
 </li>
 </ul>
 
+<h3 id="deprecated-headers">Deprecated headers</h3>
+<p>
+Some C++ Standard Library headers now produce deprecation warnings when
+included. The warnings suggest how to adjust the code to avoid the warning,
+for example all uses of <code><cstdbool></code> and
+<code><cstdalign></code> can simply be removed,
+because they serve no purpose and are unnecessary in C++ programs.
+Most uses of <code><ccomplex></code> can be adjusted to use
+<code><complex></code> instead, and uses of <code><ctgmath></code>
+can use <code><cmath></code> and/or <code><complex></code>.
+</p>
 
 <!-- <h2 id="fortran">Fortran language issues</h2> -->
 
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