diff --git a/docs/ReleaseNotes.rst b/docs/ReleaseNotes.rst
index df4ee1b5a09285797af15036c77ea5e6ad92973f..67b834af397d7d005ab3d50d8c7f221a28252554 100644
--- a/docs/ReleaseNotes.rst
+++ b/docs/ReleaseNotes.rst
@@ -63,6 +63,9 @@ New Compiler Flags
   enable it.
 - Command line "clang -O3 -flto a.c -c" and "clang -emit-llvm a.c -c" 
   are no longer equivalent.
+- Clang now errors on unknown -m flags (``-munknown-to-clang``),
+  unknown -f flags (``-funknown-to-clang``) and unknown
+  options (``-what-is-this``).
 
 C Language Changes in Clang
 ---------------------------