Add a release note entry

Author: winner245

libcxx/docs/ReleaseNotes/21.rst

@@ -61,6 +61,9 @@ Improvements and New Features
 
 - Updated formatting library to Unicode 16.0.0.
 
+- The segmented iterator optimization for ``std::for_each``, previously available only in C++23 and later, has been
+  backported to C++11 to enable consistent optimization across standards.
+
 Deprecations and Removals
 -------------------------