[clang] [clang] Add comment about misleading alloc_size argument names (PR #134899)

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thejh wrote:

cc @gburgessiv who originally introduced the attribute in https://github.com/llvm/llvm-project/commit/e37633713d93b21533d8699d0b5cf4703e3f4039

https://github.com/llvm/llvm-project/pull/134899