diff --git a/libcxx/docs/ReleaseNotes/18.rst b/libcxx/docs/ReleaseNotes/18.rst
index 80b42ad7f653aa..862284ce5494c9 100644
--- a/libcxx/docs/ReleaseNotes/18.rst
+++ b/libcxx/docs/ReleaseNotes/18.rst
@@ -124,7 +124,9 @@ Improvements and New Features
   ``-DLIBCXX_INSTALL_MODULE_DIR=<path>``. The default location is
   ``${PREFIX}/share/libc++/v1``.
 
-- AddressSanitizer annotations have been added to ``std::basic_string``.
+- AddressSanitizer annotations have been added to ``std::basic_string``
+  external buffers (long strings only).
+  Enabled with all allocators by default.
 
 - The libc++ source code has been formatted with ``clang-format``. This
   `discourse thread <https://discourse.llvm.org/t/rfc-clang-formatting-all-of-libc-once-and-for-all>`_