Add a note to release note

Author: KaiYG

clang/docs/ReleaseNotes.rst

@@ -69,6 +69,16 @@ Potentially Breaking Changes
   call the member ``operator delete`` instead of the expected global
   delete operator. The old behavior is retained under ``-fclang-abi-compat=21``
   flag.
+- Trailing null statements in GNU statement expressions are no longer
+  ignored by Clang; they now result in a void type. Clang previously
+  matched GCC's behavior, which was recently clarified to be incorrect.
+
+  .. code-block:: c++
+
+    // The resulting type is 'void', not 'int'
+    void foo(void) {
+      return ({ 1;; });
+    }
 
 C/C++ Language Potentially Breaking Changes
 -------------------------------------------