Rollup merge of #123588 - tgross35:stabilize-assert_unchecked, r=dtolnay

Stabilize `hint::assert_unchecked`

Make the following API stable, including const:

```rust
// core::hint, std::hint

pub const unsafe fn assert_unchecked(p: bool);
```

This PR also reworks some of the documentation and adds an example.

Tracking issue: https://github.com/rust-lang/rust/issues/119131
FCP: https://github.com/rust-lang/rust/issues/119131#issuecomment-1906394087. The docs update should resolve the remaining concern.

library/std/src/lib.rs

@@ -334,7 +334,6 @@
 #![feature(fmt_internals)]
 #![feature(hasher_prefixfree_extras)]
 #![feature(hashmap_internals)]
-#![feature(hint_assert_unchecked)]
 #![feature(ip)]
 #![feature(maybe_uninit_slice)]
 #![feature(maybe_uninit_write_slice)]