getNotes example

// A link relation type of the "registered" kind.     /** @var \Drupal\Core\Http\LinkRelationTypeInterface $canonical */
    $canonical = $link_relation_type_manager->createInstance('canonical');
    $this->assertInstanceOf(LinkRelationType::class, $canonical);
    $this->assertTrue($canonical->isRegistered());
    $this->assertFalse($canonical->isExtension());
    $this->assertSame('canonical', $canonical->getRegisteredName());
    $this->assertNull($canonical->getExtensionUri());
    $this->assertEquals('[RFC6596]', $canonical->getReference());
    $this->assertEquals('Designates the preferred version of a resource (the IRI and its contents).', $canonical->getDescription());
    $this->assertEquals('', $canonical->getNotes());

    // A link relation type of the "extension" kind.     /** @var \Drupal\Core\Http\LinkRelationTypeInterface $canonical */
    $add_form = $link_relation_type_manager->createInstance('add-form');
    $this->assertInstanceOf(LinkRelationType::class, $add_form);
    $this->assertFalse($add_form->isRegistered());
    $this->assertTrue($add_form->isExtension());
    $this->assertNull($add_form->getRegisteredName());
    $this->assertSame('https://drupal.org/link-relations/add-form', $add_form->getExtensionUri());
    $this->assertEquals('', $add_form->getReference());
    $this->assertEquals('A form where a resource of this type can be created.', $add_form->getDescription());