Document match statements

[Akuli]

Introduced in #617

[Akuli]

Should probably document how match statements work in general:

• match foo: syntax accepts integers and enums only (doesn't really make sense to match other types, e.g. floats)
• you will get a warning about duplicate/missing case only if you use match some_enum:

[Akuli]

Match statements are partially documented in enum documentation. Will need to make sure that there's not too much duplication.

[Akuli]

I think I should document the following as UB:

import "stdlib/io.jou"

enum Foo:
    Bar  # internally 0
    Baz  # internally 1

def main() -> int:
    f = 7 as Foo
    match f:
        case Foo.Bar:
            printf("Bar\n")
        case Foo.Baz:
            printf("Baz\n")
    return 0

Then the compiler could assume that one of the cases runs, so that if each case e.g. sets a variable or returns a value, the compiler will not warn about possibly undefined values.