Missing documentation around casting foreign non-exhaustive enums

[lambinoo]

After rust-lang/rust#91161 by rust-lang/rust#92744 got merged, the new behavior for non-exhaustive enums on cast should be documented.

use othercrate::NonExhaustiveEnum;

let _ = NonExhaustiveEnum::default() as u8; // This is a compile error now

Non exhaustive type documention is not reflecting that right now.