Add note for match guards to include catch-all

[saward]

The compiler doesn't check for conditions being exhausted when using arbitrary expressions (rust-lang/rust#74277). This adds a note with example specifying that you need to cover all remaining conditions with _.

My main concern is that my example may encourage people to use match guards as I did there, instead of something like:

    match number {
        0 => println!("Zero"),
        1..=u8::MAX => println!("Greater than zero"),
    }

Also, maybe I should just add this as a note without an example?

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @marioidival (or someone else) soon.

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