Document that programmer can't use generic NonZero

[ijackson]

Location

std::num::NonZero in the standard library rustdocs

Summary

Since #120257, we have had, on stable, NonZero<T>. But really it's NonZero<T: unnameable> (because ZeroablePrimitive is perma-unstrable).

The effect is that the programmer can talk about NonZero<SomeConcreteType>, but unlike most other generics in the standard library, they can't impl<T:...> MyTrait for NonZero<T>. (That's point 5 in "Other surface area not being proposed for stabilization right now" in #120257 (comment).)

It would be good if this were explicitly mentioned in the docs for NonZero. That might save people setting out to try to do the impossible.