[SE-289] Adjust selection statements examples to match reality (#1730)
[SE-289] Adjust selection statements examples to match reality (#1730)

Examples made it look like `buildOptional` calls are injected
after enclosing `if` when it doesn't have an else, which is
incorrect - `buildOptional` happens inside the block and `else`
is synthesized by the result builder transform.
apple/swift-evolution,apple/swift-evolution