Clarify switch expression usage

[MaryaBelanger]

dart-lang/language#3061

https://dart.dev/language/branches#switch-expressions

Change example to:

enum Color { red, blue, green }

void main() {
  final color = Color.red;
  (switch (color) {
    Color.red => "red",
    Color.blue => "blue",
    Color.green => "green",
  });
}

The solution is to clarify that they can't be used as the beginning of an expression. Then, if they really need to do that, parenthesize it like the above example. But I think instead of parenthesizing it, these cases should really just be using a switch statement. I'm hesitant to mention the parenthesized syntax since it's a work around to achieve behavior that's already built into switch statements.