Add a release note entry

Author: winner245

libcxx/docs/ReleaseNotes/21.rst

@@ -67,6 +67,9 @@ Improvements and New Features
 - The ``std::stable_sort`` algorithm uses radix sort for floating-point types now, which can improve the performance
   up to 10x, depending on type of sorted elements and the initial state of the sorted array.
 
+- The segmented iterator optimization for ``std::for_each`` has been backported to C++11. Previously it was only available
+  in C++23 and later.
+
 Deprecations and Removals
 -------------------------