Update release notes

Author: ChuanqiXu9

clang/docs/ReleaseNotes.rst

@@ -412,9 +412,9 @@ Modified Compiler Flags
   ``memset`` and similar functions for which it is a documented undefined
   behavior.
 
-- Added ``-fmodules-reduced-bmi`` flags corresponding to
-  ``-fexperimental-modules-reduced-bmi`` flag and emit warning for not emitting
-  reduced BMI.
+- Added ``-fmodules-reduced-bmi`` flag corresponding to
+  ``-fexperimental-modules-reduced-bmi`` flag. The ``-fmodules-reduced-bmi`` flag
+  is intended to be enabled by default in the future.
 
 Removed Compiler Flags
 -------------------------