Index: clang/docs/ReleaseNotes.rst
===================================================================
--- clang/docs/ReleaseNotes.rst
+++ clang/docs/ReleaseNotes.rst
@@ -110,6 +110,13 @@
 C++20 Feature Support
 ^^^^^^^^^^^^^^^^^^^^^
 
+- The mangling scheme for modules has incompatibally changed. The
+  initial mangling was discovered not to be reversible, and the weak
+  ownership design decision did not give the backwards compatibility
+  that was hoped for. C++20 since added ``extern "C++"`` semantics
+  that can be used for that. The demangler now demangles symbols with
+  named module attachment.
+
 C++2b Feature Support
 ^^^^^^^^^^^^^^^^^^^^^