[libc++][doc] Update the release notes

Differential Revision: https://reviews.llvm.org/D106770

Author: mordante

libcxx/docs/BuildingLibcxx.rst

@@ -246,10 +246,10 @@ libc++ specific options
 
 .. option:: LIBCXX_ENABLE_FILESYSTEM:BOOL
 
-   **Default**: ``ON`` except on Windows.
+   **Default**: ``ON`` except on Windows when using MSVC.
 
    This option can be used to enable or disable the filesystem components on
-   platforms that may not support them. For example on Windows.
+   platforms that may not support them. For example on Windows when using MSVC.
 
 .. option:: LIBCXX_ENABLE_INCOMPLETE_FEATURES:BOOL
 

libcxx/docs/ReleaseNotes.rst

@@ -35,10 +35,37 @@ see the `releases page <https://llvm.org/releases/>`_.
 What's New in Libc++ 13.0.0?
 ============================
 
+- Support for older compilers has been removed. Several additional platforms
+  are now officially supported. :ref:`platform_and_compiler_support` contains
+  the complete overview of platforms and compilers supported by libc++.
+- The large headers ``<algorithm>``, ``<iterator>``, and ``<utility>`` have
+  been split in more granular headers. This reduces the size of included code
+  when using libc++. This may lead to missing includes after upgrading to
+  libc++13.
+
 New Features
 ------------
 
-- ...
+- ``std::filesystem`` is now feature complete for the Windows platform using
+  MinGW. MSVC isn't supported since it lacks 128-bit integer support.
+- The implementation of the C++20 concepts library has been completed.
+- Several C++20 ``constexpr`` papers have been completed:
+
+  - `P0879R0 <https://wg21.link/P0879R0>`_ ``constexpr`` for ``std::swap()``
+    and swap related functions
+  - `P1032R1 <https://wg21.link/P1032R1>`_ Misc ``constexpr`` bits
+  - `P0883 <https://wg21.link/P0883>`_ Fixing Atomic Initialization
+
+- More C++20 features have been implemented. :doc:`Status/Cxx20` has the full
+  overview of libc++'s C++20 implementation status.
+- More C++2b features have been implemented. :doc:`Status/Cxx2b` has the
+  full overview of libc++'s C++2b implementation status.
+- The CMake option ``LIBCXX_ENABLE_INCOMPLETE_FEATURES`` has been added. This
+  option allows libc++ vendors to disable headers that aren't production
+  quality yet. Currently, turning the option off disables the headers
+  ``<format>`` and ``<ranges>``.
+- The documentation conversion from html to restructured text has been
+  completed.
 
 API Changes
 -----------

libcxx/docs/index.rst

@@ -91,6 +91,7 @@ reasons, but some of the major ones are:
   Further, both projects are apparently abandoned: STLport 5.2.1 was
   released in Oct'08, and STDCXX 4.2.1 in May'08.
 
+.. _platform_and_compiler_support:
 
 Platform and Compiler Support
 =============================