[libc++] Add details about string annotations (#80912)

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 comments in a related
PR: #79536

---------

Co-authored-by: Mark de Wever <zar-rpg@xs4all.nl>

libcxx/docs/ReleaseNotes/18.rst

@@ -125,6 +125,8 @@ Improvements and New Features
   ``${PREFIX}/share/libc++/v1``.
 
 - AddressSanitizer annotations have been added to ``std::basic_string``.
+  These annotations are enabled for all allocators by default.
+  It's only enabled for long strings, strings using the small buffer optimization are not annotated.
 
 - 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>`_