fix static analysis and doc

UPGRADE.md

@@ -52,7 +52,7 @@ Now on invoice admin and shop user can check if related order was paid before in
  Collection $lineItems,
  Collection $taxItems,
  ChannelInterface $channel,
- + bool $isPaid,
+ + string $paymentState,
  InvoiceShopBillingDataInterface $shopBillingData = null
  ): InvoiceInterface {
  // ...

src/Entity/InvoiceInterface.php

@@ -21,6 +21,7 @@
 interface InvoiceInterface extends ResourceInterface
 {
  public const PAYMENT_STATE_COMPLETED = 'completed';
+
  public const PAYMENT_STATE_PENDING = 'pending';
 
  public function id(): string;