Unstable Book: "foo" leads to an unrelated tracking issue, it's not a real feature

[ArekPiekarz]

The Unstable Book in section foo leads to a tracking issue #1234, which is about an unrelated topic rust-mac1 bot has issues. The naming of both the feature and the issue leads me to believe they are only given as examples in the code and are being incorrectly picked up by the documentation generator as real ones.

For instance they are used in library/core/src/intrinsics.rs:

//! In order to make an intrinsic usable at compile-time, one needs to copy the implementation
//! from https://github.com/rust-lang/miri/blob/master/src/shims/intrinsics.rs to
//! `librustc_mir/interpret/intrinsics.rs` and add a
//! `#[rustc_const_unstable(feature = "foo", issue = "01234")]` to the intrinsic.