[libcxx] Add release notes for Windows wide stdio stream handling

[mstorsjo]

Summary

This adds notes for the change from https://reviews.llvm.org/D146398 /
fcbbd9649ac165aaf7fc7d60b8fef3b23755179a.

[Mordante]

LGTM modulo one nit.

libcxx/docs/ReleaseNotes/17.rst

115

libcxx/docs/ReleaseNotes/17.rst

- The library now provides a debug mode which is a superset of the hardened mode, additionally enabling more expensive
   whether the debug mode is enabled by default with the ``LIBCXX_HARDENING_MODE`` variable at CMake configuration time.
   Users can control whether the debug mode is enabled on a per translation unit basis using the
   ``-D_LIBCPP_ENABLE_DEBUG_MODE=1`` macro. See ``libcxx/docs/Hardening.rst`` for more details.
 
 - ASan container annotations have been extended to cover all allocators in ``std::vector``.
 
 - ASan annotations have been added to the ``std::deque`` container, to detect container overflows.
 
+- On Windows, ``std::wcout``, ``wcin``, and ``wcerr`` now work correctly when the underlying
+  stream has been configured in wide mode.
+
 Deprecations and Removals
 -------------------------
 
 - The "safe" mode is replaced by the hardened mode in this release. The ``LIBCXX_ENABLE_ASSERTIONS`` CMake variable is
   deprecated and setting it will trigger an error; use ``LIBCXX_HARDENING_MODE`` instead. Similarly, the
   ``_LIBCPP_ENABLE_ASSERTIONS`` macro is deprecated and setting it to ``1`` now enables the hardened mode. See
   ``libcxx/docs/Hardening.rst`` for more details.