Rebased and release note added for fill_n

Author: NoumanAmir657

libcxx/docs/ReleaseNotes/20.rst

@@ -51,6 +51,9 @@ Improvements and New Features
 
 - ``std::fill`` has been optimized for segmented iterators like ``std::deque::iterator`` in C++23 and
   later, which can lead up to 40x performance improvements.
+  
+- ``std::fill_n`` has been optimized for segmented iterators like ``std::deque::iterator`` in C++23 and
+  later, which can lead up to 40x performance improvements.
 
 - The ``_LIBCPP_ENABLE_CXX20_REMOVED_UNCAUGHT_EXCEPTION`` macro has been added to make ``std::uncaught_exception`` available in C++20 and later modes.