[wwwdocs] gcc-13/changes.html: Document C++ -fexcess-precision=standard

[Jakub Jelinek <jakub@redhat.com>]

Hi!

Martin Liska mentioned that porting_to.html doesn't mention
the C++ excess precision changes.  Not really sure if porting_to
should document those, but I think changes.html certainly should.
And perhaps porting_to could have some examples and link to this
spot in changes.

Ok for wwwdocs?

--- a/htdocs/gcc-13/changes.html
+++ b/htdocs/gcc-13/changes.html
@@ -230,6 +230,21 @@ a work-in-progress.</p>
 
 <h3 id="cxx">C++</h3>
 <ul>
+  <li>Excess precision support (which has been available in C since GCC 4.5)
+      has been implemented for C++ as well.  It is enabled by default
+      in strict standard modes like <code>-std=c++17</code>, where it
+      defaults to <code>-fexcess-precision=standard</code>, while in GNU
+      standard modes like <code>-std=gnu++20</code> it defaults to
+      <code>-fexcess-precision=fast</code>.  The option affects mainly
+      IA-32/x86-64 where when defaulting to x87 math and in some cases on
+      Motorola 68000 <code>float</code> and <code>double</code> expressions
+      are evaluated in <code>long double</code> precision and S/390, System z,
+      IBM z Systems where <code>float</code> expressions are evaluated in
+      <code>double</code> precision.  Also, on several architectures where
+      <code>std::float16_t</code> or <code>std::bfloat16_t</code> types
+      are supported those are evaluated in <code>float</code> precision.
+      <code>-fexcess-precision=fast</code> restores previous behavior.
+  </li>
   <li>Several C++23 features have been implemented:
     <ul>
       <li><a href="https://wg21.link/p2324">P2324R1</a>, Labels at the end of

	Jakub