Rollup merge of #133942 - BD103:black-box-docs, r=saethlin

Clarify how to use `black_box()`

Closes #133923.

r? libs
^ (I think that's the right group, this is my first time!)

This PR adds further clarification on the [`black_box()`](https://doc.rust-lang.org/stable/std/hint/fn.black_box.html) documentation. Specifically, it teaches _how_ to use it, instead of just _when_ to use it.

I tried my best to make it clear and accurate, but a lot of my information is sourced from rust-lang/rust-clippy#12707 and [manually inspecting assembly](https://godbolt.org/). Please tell me if I got anything wrong!

Author: matthiaskrgr