[libc++][doc] Update the release notes for LLVM 20. (#124403)

This is a preparation for the upcoming LLVM 20 release.

libcxx/docs/ReleaseNotes/20.rst

@@ -35,6 +35,8 @@ see the `releases page <https://llvm.org/releases/>`_.
 What's New in Libc++ 20.0.0?
 ==============================
 
+The main focus of the libc++ team has been to implement new C++20, C++23, and C++26 features.
+
 Implemented Papers
 ------------------
 
@@ -44,7 +46,7 @@ Implemented Papers
 - P2985R0: A type trait for detecting virtual base classes (`Github <https://github.com/llvm/llvm-project/issues/105432>`__)
 - ``std::jthread`` and ``<stop_token>`` are not guarded behind ``-fexperimental-library`` anymore
 - P2674R1: A trait for implicit lifetime types (`Github <https://github.com/llvm/llvm-project/issues/105259>`__)
-- P0429R9: A Standard ``flat_map`` is partially implemented and ``flat_map`` is provided (`Github <https://github.com/llvm/llvm-project/issues/105190>`__)
+- P0429R9: A Standard ``flat_map`` (`Github <https://github.com/llvm/llvm-project/issues/105190>`__)
 
 Improvements and New Features
 -----------------------------
@@ -111,6 +113,13 @@ Improvements and New Features
   std::errc::not_a_directory``, or use ``err.default_error_condition()`` to map to an ``error_condition``, and then test
   its ``value()`` and ``category()``.
 
+- ``std::stable_sort`` uses radix sort for integral types now, which can improve the performance up to 10 times, depending
+  on type of sorted elements and the initial state of the sorted array.
+
+- Reduced the amount of debug information generated for internal typedefs. This reduces the size of debug builds.
+
+- Added :ref:`hardening mode <hardening>` support for ``forward_list`` and ``bitset``.
+
 Deprecations and Removals
 -------------------------
 
@@ -129,10 +138,10 @@ Deprecations and Removals
   supported as an extension anymore, please migrate any code that uses e.g. ``std::vector<const T>`` to be
   standards conforming.
 
-- Non-conforming member typedefs ``base``, ``iterator``, ``const_iterator``, and ``const_reference`` of ``std::bitset``,
-  and member typedef ``base`` of ``std::forward_list`` and ``std::list`` are removed. Previously, these member typedefs
-  (except ``const_reference``) were private but could cause ambiguity in name lookup. Code that expects such ambiguity
-  will possibly not compile in LLVM 20.
+- Non-conforming member typedefs ``base``, ``iterator``, ``const_iterator``, ``size_type``, ``difference_type``, and
+  ``const_reference`` of ``std::bitset``, and member typedef ``base`` of ``std::forward_list`` and ``std::list`` are
+  removed. Previously, these member typedefs (except ``const_reference``) were private but could cause ambiguity in name
+  lookup. Code that expects such ambiguity will possibly not compile in LLVM 20.
 
 - The function ``__libcpp_verbose_abort()`` is now ``noexcept``, to match ``std::terminate()``. (The combination of
   ``noexcept`` and ``[[noreturn]]`` has special significance for function effects analysis.) For backwards compatibility,
@@ -196,8 +205,3 @@ ABI Affecting Changes
 
 - The localization support base API has been reimplemented, leading to different functions being exported from the
   libc++ built library on Windows and Windows-like platforms.
-
-Build System Changes
---------------------
-
-- TODO

libcxx/docs/Status/Cxx23Papers.csv

@@ -52,7 +52,7 @@
 "`P2443R1 <https://wg21.link/P2443R1>`__","``views::chunk_by``","2022-02 (Virtual)","|Complete|","18",""
 "","","","","",""
 "`P0009R18 <https://wg21.link/P0009R18>`__","mdspan: A Non-Owning Multidimensional Array Reference","2022-07 (Virtual)","|Complete|","18",""
-"`P0429R9 <https://wg21.link/P0429R9>`__","A Standard ``flat_map``","2022-07 (Virtual)","|Complete|","",""
+"`P0429R9 <https://wg21.link/P0429R9>`__","A Standard ``flat_map``","2022-07 (Virtual)","|Complete|","20",""
 "`P1169R4 <https://wg21.link/P1169R4>`__","``static operator()``","2022-07 (Virtual)","|Complete|","16",""
 "`P1222R4 <https://wg21.link/P1222R4>`__","A Standard ``flat_set``","2022-07 (Virtual)","","",""
 "`P1223R5 <https://wg21.link/P1223R5>`__","``ranges::find_last()``, ``ranges::find_last_if()``, and ``ranges::find_last_if_not()``","2022-07 (Virtual)","|Complete|","19",""