docs: static Watch forDocument(DocumentReference documentReference) {
      Target.Builder target = Target.newBuilder();
      target.getDocumentsBuilder().addDocuments(documentReference.getName());
      target.setTargetId(WATCH_TARGET_ID);
  
      return new Watch(
          (FirestoreImpl) documentReference.getFirestore(),
          documentReference.getParent(),
          target.build());
    }