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

[Chuanqi Xu via cfe-commits]

https://github.com/ChuanqiXu9 approved this pull request.

I like the idea. There are a lot of people suffering not able to send issue reports for modules.

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