Rollup merge of #146924 - cptpiepmatz:doc-nonzero-const-creation, r=joboet

Add doc for `NonZero*` const creation

I ran into trouble using `NonZero*` values because I didn’t see any clear way to create them at compile time. At first I ended up using `NonZero*::new_unchecked` a lot, until I realized that `Option::unwrap` and `Option::expect` are `const` and can be used in a `const` context. With that, you can create non-zero values at compile time safely, without touching `unsafe`. This wasn’t obvious to me and my peers who’ve been using Rust for a while, so I thought adding a note to the docs would make it easier for others to discover.

If this should be worded differently or placed in another location, we can do that. I just want to make this more obvious.

Author: matthiaskrgr