Add release note to this optimization in 20.rst

Author: winner245

libcxx/docs/ReleaseNotes/20.rst

@@ -69,6 +69,10 @@ Improvements and New Features
 - The ``_LIBCPP_ABI_BOUNDED_ITERATORS_IN_STD_ARRAY`` ABI configuration was added, which allows storing valid bounds
   in ``std::array::iterator`` and detecting OOB accesses when the appropriate hardening mode is enabled.
 
+- The `assign(_InputIterator, _InputIterator)` function of `std::vector<_Tp, _Allocator>` has been optimized for
+  non-trivial element types, such as `std::vector<std::string>`, with a performance improvement of up to 2.3x. The
+  performance for trivial types, such as `std::vector<int>`, remains similar or shows slight improvements.
+
 Deprecations and Removals
 -------------------------