Update release notes

Author: spaits

clang/docs/ReleaseNotes.rst

@@ -70,7 +70,9 @@ C++2c Feature Support
 
 Resolutions to C++ Defect Reports
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-
+- Substitute template parameter pack, when it is not explicitly specified
+  in the template parameters, but is deduced from a previous argument.
+  (`#78449: <https://github.com/llvm/llvm-project/issues/78449>`_).
 C Language Changes
 ------------------