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



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