[libc++] Add details about string annotations (#82730)

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:
https://github.com/llvm/llvm-project/pull/79536

Upstreamed in: 7661ade5


Upstream PR: #80912

---------

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

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