[wwwdocs] Document <cstdint> changes in libstdc++

[Jonathan Wakely <jwakely@redhat.com>]

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Pushed to wwwdocs.

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commit f55f35c86c68143a2b148c66e4b0b560c852ce6f
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue May 24 13:06:11 2022 +0100

    Document <cstdint> changes in libstdc++

diff --git a/htdocs/gcc-13/porting_to.html b/htdocs/gcc-13/porting_to.html
index b3e0895a..84a00f21 100644
--- a/htdocs/gcc-13/porting_to.html
+++ b/htdocs/gcc-13/porting_to.html
@@ -24,5 +24,23 @@ porting to GCC 13. This document is an effort to identify common issues
 and provide solutions. Let us know if you have suggestions for improvements!
 </p>
 
+<h2 id="cxx">C++ language issues</h2>
+
+<h3 id="header-dep-changes">Header dependency changes</h3>
+<p>Some C++ Standard Library headers have been changed to no longer include
+other headers that were being used internally by the library.
+As such, C++ programs that used standard library components without
+including the right headers will no longer compile.
+</p>
+<p>
+The following headers are used less widely in libstdc++ and may need to
+be included explicitly when compiled with GCC 13:
+</p>
+<ul>
+<li> <code>&lt;cstdint&gt;</code>
+  (for <code>std::int8_t</code>, <code>std::int32_t</code> etc.)
+</li>
+</ul>
+
 </body>
 </html>