code: @Nonnull
    public T delete(
        @Nonnull DocumentReference documentReference, @Nonnull Precondition precondition) {
      return performDelete(documentReference, precondition);
    }
docs: Deletes the document referred to by this DocumentReference.
  
  @param documentReference The DocumentReference to delete.
  @param precondition Precondition for the delete operation.
  @return The instance for chaining.