Polish CoreML Llama Doc (#5745)

Summary:
Document
* The export python script arguments to export llama for coreml
* The cmake argument to compile Mac runner with coreml backend

Minor fix: Remove outdated arguments

Pull Request resolved: https://github.com/pytorch/executorch/pull/5745

Reviewed By: kirklandsign

Differential Revision: D63652220

Pulled By: cccclai

fbshipit-source-id: 1703158b4e2b31cc29d08e9262e56d564a90e539