[clang] [clang] C++20 Modules: document how to perform automated reductions (PR #124997)

[Michael Spencer via cfe-commits]

https://github.com/Bigcheese approved this pull request.

While I agree it would be good to full documentation on how to reduce test cases, I think this subset is very useful for modules issues.

I think this is fine with or without the changes I suggested.

https://github.com/llvm/llvm-project/pull/124997