[clang] [rtsan][NFC] Add 'known to have non-deterministic' caveat to documentation (PR #114281)

[Chris Apple via cfe-commits]

cjappl wrote:

(doc build failure is unrelated, I will rebase after approval and make sure that passes):

> Warning, treated as error:
> /home/runner/work/llvm-project/llvm-project/clang-build/tools/clang/docs/index.rst:89:toctree contains reference to nonexisting document 'ClangFormattedStatus'

https://github.com/llvm/llvm-project/pull/114281