[wwwdocs] Document new libstdc++ features

[Jonathan Wakely]

Pushed to wwwdocs.


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commit 86937d6dfe04c81c805cfc321c62a520dfd2a59f
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Feb 25 12:54:45 2021 +0000

    Document new libstdc++ features

diff --git a/htdocs/gcc-11/changes.html b/htdocs/gcc-11/changes.html
index de75b8d6..6a47b0b8 100644
--- a/htdocs/gcc-11/changes.html
+++ b/htdocs/gcc-11/changes.html
@@ -429,12 +429,15 @@ a work-in-progress.</p>
 <ul>
   <li>Improved C++17 support, including:
     <ul>
-      <li><code>std::from_chars</code> for floating-point types.</li>
+      <li><code>std::from_chars</code> and <code>std::to_chars</code>
+        for floating-point types.</li>
     </ul>
   </li>
   <li>Improved experimental C++20 support, including:
     <ul>
-      <li>Calendar additions to <code><chrono></code>.</li>
+      <li>Calendar additions to <code><chrono></code>.
+        Thanks to Cassio Neri for optimizations.
+      </li>
       <li><code>std::bit_cast</code></li>
       <li><code>std::source_location</code></li>
       <li>Atomic wait and notify operations.</li>
@@ -449,6 +452,7 @@ a work-in-progress.</p>
       <li><code>contains</code> member functions for strings,
         thanks to Paul Fee.
       </li>
+      <li><code>std::to_underlying</code></li>
     </ul>
   </li>
   <li>Experimental support for Data-Parallel Types (<code>simd</code>)