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/core/src/lib.rs

@@ -127,7 +127,6 @@
 #![feature(const_fmt_arguments_new)]
 #![feature(const_hash)]
 #![feature(const_heap)]
-#![feature(const_hint_assert_unchecked)]
 #![feature(const_index_range_slice_index)]
 #![feature(const_int_from_str)]
 #![feature(const_intrinsic_copy)]