Add a note that ! can only be coerced to sized types

[WaffleLapkin]

This commit is dated 2025-02-02, I accidentally found it while looking for something else lol.

[traviscross]

Do you happen to have an example of something that would work but for this?

[WaffleLapkin]

Checking again, it doesn't seem like this restriction is present in rustc (anymore?). Indeed this program (using incomplete features though!) compiles:

#![feature(unsized_locals)]

fn main() {
    let x: [()] = panic!();
    f(&x);
}

fn f(_: &[()]) {}

In practice though you can't make code that would do this coercion, simply because all coercion sites require sized types.