Grouping of API methods in documentation – is there some custom attribute

You could also use SwaggerOperationAttribute:

public class UserController : ApiController
{
    [Route("api/user")]
    [SwaggerOperation(Tags = new[] { "User" })]
    IHttpActionResult GetUser() { ... }
}

public class ResumeController : ApiController
{
    [Route("api/user/resumes")]
    [SwaggerOperation(Tags = new[] { "User" })]
    IHttpActionResult GetResumes() { ... }
}