[clang] Revise the modules document for clarity (PR #90237)

[Vlad Serebrennikov via cfe-commits]

https://github.com/Endilll commented:

I have to admit that is rather shallow due to sheer amount of text, but it still took me more like 90 minutes. Hope you find it useful. 

https://github.com/llvm/llvm-project/pull/90237