[Notifier] Add more specific types at documentation level when php engine can't

Author: Philippe Segatori

Message/SmsMessage.php

@@ -58,7 +58,7 @@ public function getPhone(): string
         return $this->phone;
     }
 
-    public function getRecipientId(): ?string
+    public function getRecipientId(): string
     {
         return $this->phone;
     }

Transport/NullTransportFactory.php

@@ -20,6 +20,9 @@
  */
 final class NullTransportFactory extends AbstractTransportFactory
 {
+    /**
+     * @return NullTransport
+     */
     public function create(Dsn $dsn): TransportInterface
     {
         if ('null' === $dsn->getScheme()) {