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

This adds notes for the change from https://reviews.llvm.org/D146398 /
fcbbd96.

Differential Revision: https://reviews.llvm.org/D156627

(cherry picked from commit 9abc6d9)

Author: mstorsjo

libcxx/docs/ReleaseNotes/17.rst

@@ -112,6 +112,9 @@ Improvements and New Features
 
 - 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
 -------------------------