C ffi docs update

[lygstate]

No functional change, just document update

[rustbot]

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[rustbot]

⚠️ #[rustc_intrinsic_const_stable_indirect] controls whether intrinsics can be exposed to stable const
code; adding it needs t-lang approval.

cc @rust-lang/wg-const-eval

[lygstate]

@ricci009 take a look at this, I think this may simplify the MR #133944

[Mark-Simulacrum]

IMO this macro isn't adding enough value to be a macro. It's ~only saving on the repetition of the doc(cfg(all())), but that's not hard to copy/paste across all of these (and why is it needed?)

[lygstate]

maybe we can drop the macro now？ don't know why it introduced at the first place, maybe it's not needed anymore?

[Mark-Simulacrum]

If you'd like to revise this PR to not move things around and keep the old structure, I'd be happy to merge it with just the new docs. (I think that means just keeping the first commit). The separate files are intentional per #136334, though we haven't (yet) made use of that, so I'd prefer to avoid significant refactoring in the shape for now.