When you've got an entity with a ApiFilter, Hydra will give some information about all fields with a filter. In this simple example it will give the name of the field (createdAt) but that's all:

class Entity
{
  #[ApiFilter(DateFilter::class, properties: ['createdAt'])]
  protected ?DateTimeInterface $createdAt = null;
}

'hydra:search': { 
  "@type": "hydra:IriTemplate"
  "hydra:mapping": {
    "@type": "IriTemplateMapping",
    "variable": "createdAt[after]",
    "property": "createdAt",
    "required": false
  }
}

This information can be used to populate search fields client side, like Swagger does. But what if I'd like to add more information about a field?