[libc++] Add details about string annotations

This commit adds information that only long strings are annotated,
and with all allocators by default.

To read why short string annotations are not turned on yet, read
related PR: llvm#79536

Author: Advenam Tacet

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>`_