[clang] [Doc] Update documentation for no-transitive-change (PR #96453)

[Ilya Biryukov via cfe-commits]

ilya-biryukov wrote:

> > I really like the idea of documenting this, hopefully this create some fruitful discussions with the build system developers and other compiler users.
> > After trying to put myself into the shoes of the compiler users reading this, I can't help but think that the "no transitive changes" is slightly confusing and hard to grasp. The example does help quite a bit, but I also feel there should be a way to document this in shorter form. (E.g. we can double-down on the "this is a build optimization" angle and being explicit that we want people to try this and let them know how to report bugs about it?)
> > I wanted to take a little bit of time to think how to do it before providing concrete suggestions, but still wanted to mention this.
> 
> How about adding a sentence like "this section is primarily for build system vendors. For end compiler users, if you don't want to read it all, this is helpful to reduce recompilations. We encourage build system vendors and end users try this out and bring feedbacks"?

Yeah, SG, I'd also recommend adding something like that to the release notes (I suspect many more people will read it initially when Clang 19 is released than the section on C++20 Modules)

https://github.com/llvm/llvm-project/pull/96453