[wwwdocs] Document PSTL and std::visit support

[Jonathan Wakely]

We still need to document PSTL in the libstdc++ manual, but this adds
it to the release notes.

Committed to CVS.


Index: htdocs/gcc-9/changes.html
===================================================================
RCS file: /cvs/gcc/wwwdocs/htdocs/gcc-9/changes.html,v
retrieving revision 1.57
diff -u -r1.57 changes.html
--- htdocs/gcc-9/changes.html	2 Apr 2019 14:11:22 -0000	1.57
+++ htdocs/gcc-9/changes.html	5 Apr 2019 18:17:28 -0000
@@ -568,8 +568,14 @@
 
 <h4 id="libstdcxx">Runtime Library (libstdc++)</h4>
 <ul>
-  <li>Improved experimental support for C++17,
-      including <code>&lt;memory_resource&gt;</code>.
+  <li>Improved experimental support for C++17, including:
+      <ul>
+      <li>
+        Parallel algorithms and <code>&lt;execution&gt;</code>
+        (requires TBB 2018 or newer).
+      </li>
+      <li> <code>&lt;memory_resource&gt;</code>. </li>
+      </ul>
   </li>
   <li>Improved experimental support for C++2a,
       including:
@@ -587,6 +593,7 @@
       <li>Functions <code>std::midpoint</code> and <code>std::lerp</code> for
         interpolation.</li>
       <li><code>std::bind_front</code>.</li>
+      <li><code>std::visit&lt;R&gt;</code>.</li>
       <li><code>std::assume_aligned</code>.</li>
       <li>Uses-allocator construction utilities.</li>
       <li><code>std::pmr::polymorphic_allocator&lt;std::byte&gt;</code>.</li>