[libcxx] [doc] Add a release note for distributors about MinGW builds and dllimport

This allows distributors to simplify how libc++ is built in MinGW
configurations.

Differential Revision: https://reviews.llvm.org/D127151

NOKEYCHECK=True
GitOrigin-RevId: 8ab2c384b8ad5afe4f23d02961ed7327d491aaf9
diff --git a/docs/ReleaseNotes.rst b/docs/ReleaseNotes.rst
index 2042b35..8dc8733 100644
--- a/docs/ReleaseNotes.rst
+++ b/docs/ReleaseNotes.rst
@@ -184,3 +184,8 @@
 - The ``LIBCXX_ENABLE_DEBUG_MODE_SUPPORT`` CMake configuration is not supported anymore. If you
   were disabling support for the debug mode with that flag, please use ``LIBCXX_ENABLE_BACKWARDS_COMPATIBILITY_DEBUG_MODE_SYMBOLS=OFF``
   instead.
+
+- MinGW DLL builds of libc++ no longer use dllimport in their headers, which
+  means that the same set of installed headers works for both DLL and static
+  linkage. This means that distributors finally can build both library
+  versions with a single CMake invocation.