Remove note about transmute for float bitpatterns.

This particular usecase has been safely abstracted in these `std` functions: [f32::to_bits](https://doc.rust-lang.org/std/primitive.f32.html#method.to_bits), [f32::from_bits](https://doc.rust-lang.org/std/primitive.f32.html#method.from_bits), [f64::to_bits](https://doc.rust-lang.org/std/primitive.f64.html#method.to_bits), [f64::from_bits](https://doc.rust-lang.org/std/primitive.f64.html#method.from_bits). So, I think we shouldn't recommend an unnecessary use of `unsafe` here anymore.

Rollup of 15 pull requests

Successful merges:

 - #59680 (Document the -Z flag to the rustc book)
 - #59711 (Add back the substring test)
 - #59806 (compiletest: Improve no_prefer_dynamic docs)
 - #59809 (Make trait_methods_not_found use a lock)
 - #59811 (Kill dead code dominator code.)
 - #59814 (Fix broken links on std::boxed doc page)
 - #59821 (improve unknown enum variant errors)
 - #59831 (Remove strange formatting in `Ordering` docs.)
 - #59836 (std::ops::Div examples: correct nominator to numerator)
 - #59857 (SGX target: fix cfg(test) build)
 - #59876 (Update TRPL to use mdbook 0.2)
 - #59880 (Remove note about transmute for float bitpatterns.)
 - #59889 (Update diagnostics.rs)
 - #59891 (Fix the link to sort_by_cached_key)
 - #59894 (save-analysis: Pull associated type definition using `qpath_def`)

Failed merges:

r? @ghost