[libc++] Implement P1165R1 (Make stateful allocator propagation more consistent)

Reviewed By: Quuxplusone, ldionne, #libc

Spies: libcxx-commits

Differential Revision: https://reviews.llvm.org/D119112

NOKEYCHECK=True
GitOrigin-RevId: 1cfa4857693b405d21272414442b635d9678916e
diff --git a/docs/ReleaseNotes.rst b/docs/ReleaseNotes.rst
index c57a7de..1e24ad6 100644
--- a/docs/ReleaseNotes.rst
+++ b/docs/ReleaseNotes.rst
@@ -39,6 +39,7 @@
 ------------
 
  - Implemented P0627R6 (Function to mark unreachable code)
+ - Implemented P1165R1 (Make stateful allocator propagation more consistent for operator+(basic_string))
 
 API Changes
 -----------