See org.hibernate.search.v6poc.entity.orm.mapping.HibernateOrmMappingContributor.

CAUTION: When the @DocumentId is the @Id, use the provided ID in priority and only if it's missing, unproxy the entity and get the ID;
when the @DocumentId is NOT the @Id, always ignore the provided ID. See org.hibernate.search.engine.impl.WorkPlan.PerClassWork.extractProperId(Work) in Search 5.