Add a blurb to the release notes

Author: mysterymath

clang/docs/ReleaseNotes.rst

@@ -341,6 +341,11 @@ Attribute Changes in Clang
 - New format attributes ``gnu_printf``, ``gnu_scanf``, ``gnu_strftime`` and ``gnu_strfmon`` are added
   as aliases for ``printf``, ``scanf``, ``strftime`` and ``strfmon``. (#GH16219)
 
+- New attribute ``modular_format`` to allow dynamically selecting at link time
+  which aspects of a statically linked libc's printf (et al) implementation are
+  required. This can reduce code size without requiring e.g. multilibs for
+  printf features. Requires cooperation with the libc implementation.
+
 Improvements to Clang's diagnostics
 -----------------------------------
 - Diagnostics messages now refer to ``structured binding`` instead of ``decomposition``,