Remove doc of unstable feature of never type

[hehaoqian]

Remove unstable feature of never type from doc

---

https://doc.rust-lang.org/reference/types/never.html

Gives example which is unstable

let x: ! = panic!();
// Can be coerced into any type.
let y: u32 = x;

But not giving example for what has been stabilized

extern "C" {
    pub fn abort() -> !;
}

This is really confusing for newcomers:

1. The Rust reference does not mention unstable feature, except this one
2. The doc says

NB. The never type was expected to be stabilized in 1.41, but due to some last minute regressions detected the stabilization was temporarily reverted. The ! type can only appear in function return types presently. See the tracking issue for more details.

Some people stops reading when they realize never type is unstable, (such as me)
not seeing The ! type can only appear in function return types presently,
so they do not realize ! can be used in function return type in stable Rust,
unless they have read https://doc.rust-lang.org/stable/rust-by-example/fn/diverging.html

I suggest to make the doc consistent, only show stabilized stuffs

[ehuss]

Thanks! At the time we always thought ! was just around the corner, but it's still struggling to make much progress.