[libc++] Updating release notes for libc++ 21 to include a bullet about std::ranges::iota and a link to #68494

Author: jamesETsmith

libcxx/docs/ReleaseNotes/21.rst

@@ -45,6 +45,7 @@ Implemented Papers
 - P2562R1: ``constexpr`` Stable Sorting (`Github <https://github.com/llvm/llvm-project/issues/105360>`__)
 - P1222R4: A Standard ``flat_set`` is partially implemented and ``flat_set`` is provided (`Github <https://github.com/llvm/llvm-project/issues/105193>`__)
 - P0472R3: Put std::monostate in <utility> (`Github <https://github.com/llvm/llvm-project/issues/127874>`__)
+- P2440R1: Implemented ``std::ranges::iota`` (`Github <https://github.com/llvm/llvm-project/pull/68494>`__)
 
 Improvements and New Features
 -----------------------------