Each method must have a doc block.

The interface must have proper dock blocks with the existing types and return type.

For example:

<?php 
/**
 * Product type provider
 *
 * Copyright © 2015 Magento. All rights reserved.
 * See COPYING.txt for license details.
 */
namespace Magento\Catalog\Api;

/**
 * @api
 */
interface ProductTypeListInterface
{
    /**
     * Retrieve available product types
     *
     * @return \Magento\Catalog\Api\Data\ProductTypeInterface[]
     */
    public function getProductTypes();

    /**
     * Retrieve available tests
     *
     * @return \Vendor\Module\Api\Data\TestInterface[]
     */
    public function getTests();
}

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