Add clarification about perpelexity discrepancy (#6053)

Summary:
Stack from [ghstack](https://github.com/ezyang/ghstack) (oldest at bottom):
* __->__ https://github.com/pytorch/executorch/issues/6053

Pull Request resolved: https://github.com/pytorch/executorch/pull/6053

Reviewed By: mergennachin

Differential Revision: D64120330

Pulled By: helunwencser

fbshipit-source-id: 61ccaf98c4a870863ceb0b3de32e057624bfa7bd