Per @eddyb's suggestion (rust-lang/reference#652 (comment)):

Not sure these should be documented, we've been meaning to remove all 3 for years...
cc @oli-obk at least for intrinsics we could maybe have an intermediary stage where we do #[rustc_intrinsic] extern "Rust" {...} instead of extern "rust-intrinsic" {...}?

I would maybe call them "pseudo-ABIs", in that they use the ABI string to indicate something special not purporting to the ABI, but are otherwise the "Rust" ABI.

cc @oli-obk, @nagisa, and @nikomatsakis