[llvm] [LoopDist] Update the pragma info of loop distribute, NFC (PR #69825)

[via llvm-commits]

vfdff wrote:

Thanks for your review.
Currently, only `#pragma clang loop distribute(enable)` takes effect. For details, see.https://gcc.godbolt.org/z/1Tjfs8s45
No warning and error are reported for the incorrect usage, so what does `break existing code` mean?

https://github.com/llvm/llvm-project/pull/69825