API Platform - Custom controller / action description annotations not working

After some experimenting, I found that the openapi_context annotation attribute does indeed seems to ignore the response documentation. It does however allow you to provide the request body description that you are missing:

#[ApiResource(
    collectionOperations: [
        'test' => [
            'method' => 'POST',
            'path' => '/test',
            'openapi_context' => [
                'summary' => 'The endpoint summary',
                'description' => 'The endpoint description',
                'requestBody' => [
                    'description' => 'The endpoint request body description', // This one
                    'content' => [
                        'application/json' => [
                            'schema' => [
                                '$ref' => '#/components/schemas/MyResource-some.group'
                            ],
                        ],
                    ],
                ],
            ],
        ]
    ],
)]

I'm using PHP 8.0.3 and API Platform 2.6.3 while writing this, changing the annotations to docblocks should result in the same outcome for you though.

In order to document the endpoint response specification however, I had to implement a custom OpenApiFactoryInterface:

<?php declare(strict_types = 1);

namespace App;

use ApiPlatform\Core\OpenApi\Factory\OpenApiFactoryInterface;
use ApiPlatform\Core\OpenApi\Model\Operation;
use ApiPlatform\Core\OpenApi\Model\PathItem;
use ApiPlatform\Core\OpenApi\Model\RequestBody;
use ApiPlatform\Core\OpenApi\OpenApi;
use ArrayObject;
use UnexpectedValueException;

class MyResourceOpenApiFactory implements OpenApiFactoryInterface
{
    private OpenApiFactoryInterface $openApiFactory;

    public function __construct(OpenApiFactoryInterface $openApiFactory)
    {
        $this->openApiFactory = $openApiFactory;
    }

    public function __invoke(array $context = []): OpenApi
    {
        $openApi = ($this->openApiFactory)($context);
        $components = $openApi->getComponents();
        $schemas = $components->getSchemas();
        if (null === $schemas) {
            throw new UnexpectedValueException('Failed to obtain OpenApi schemas');
        }

        $pathItem = new PathItem(
            'MyResource test endpoint',
            'A test summary',
            'A test description',
            null,
            null,
            // Your custom post operation
            new Operation(
                'testMyResourceCollection', // the operation route name
                [
                    'MyResource' // your resource name
                ],
                [
                    // response specifications
                    '201' => [
                        'description' => 'test endpoint 201 response description',
                        'content' => [
                            'application/json' => [
                                'schema' => [
                                    '$ref' => '#/components/schemas/MyResource-read', // your resource (read) schema
                                ],
                            ],
                        ],
                    ],
                ],
                'A test endpoint summary',
                'A test endpoint description',
                null,
                [],
                new RequestBody(
                    'A test request body description',
                    new ArrayObject([
                        'application/json' => [
                            'schema' => [
                                '$ref' => '#/components/schemas/MyResource-write', // your resource (write) schema
                            ],
                        ],
                    ]),
                ),
            ),
        );

        $paths = $openApi->getPaths();
        $paths->addPath('/my_resources/test', $pathItem);
        return $openApi;
    }
}

Have this OpenApiFactoryInterface implementation wired by the service container as a decorator to api_platform.openapi.factory:

App\MyResourceOpenApiFactory:
    decorates: 'api_platform.openapi.factory'
    autoconfigure: false

Change the references to the example MyResource name to a resource name of your choice (like User).

Sidenote: This whole process of customizing OpenApi endpoint documentation in API Platform is currently quite convoluted in my opinion. Use the implementation I provided as a reference to your own implementation, as you most likely need to make a few adjustments to it in order to make it satisfy your specific use case.