[clang] [clang][docs] Revise documentation for `__builtin_reduce_(max|min)`. (PR #114637)

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c8ef wrote:

I apologize for my earlier mistake. The function `__builtin_reduce_max` behaves differently from `__builtin_reduce_maximum` when working with floating point numbers. The former will only return nan if all elements are nan, while the latter will return nan if any element in the vector is nan. The most recent patch corrects the wording error and restores the nan semantics. Please take another look.
@arsenm @RKSimon 

https://github.com/llvm/llvm-project/pull/114637