[clang] Docs: clarify standard library usage with C++ modules (PR #173636)

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Viditgupta-official wrote:

> That's obvious. Is it really necessary?

I agree this is obvious to experienced users. The intent here is to make an implicit assumption explicit for readers who are new to C++ modules, especially those coming from a header-based mental model. I’ve seen confusion around whether importing a C++ module implies the same guarantees as including standard headers, particularly depending on how the standard library is built and exposed.

If maintainers feel this clarification doesn’t add enough value, I’m happy to remove or tighten it.

https://github.com/llvm/llvm-project/pull/173636