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

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

> @cor3ntin do you sustain an objection due to the lack of a full beginner-friendly test case reduction tutorial?

I think this PR is improvement over status quo, but I see where Corentin is coming from with his objection. Maybe the middle ground would be to explicitly acknowledge somewhere that full tutorial is yet to be written, so that less people are confused whether such tutorial exists elsewhere in our documentation?

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