Issue Description:

The documentation of switch says that it must be exhaustive, but what does that mean is not defined in the docs as far as I can tell.

For example, this one is exhaustive:

let n = [-1, 1].randomElement()!

switch n {
case Int.min..<0:
    print("negative")
case 0...Int.max:
    print("non-negative")
}

but it does not compile. Neither does

let n = [-1, 1].randomElement()!

switch n {
case n where n < 0:
    print("negative")
case n where n >= 0:
    print("non-negative")
}

Of course, you can try and add a dummy default if the compiler does not handle that case. But in any case my point is that, formally, ideally, what exhaustive means should be in the language reference.