code: @Nonnull
    public T update(
        @Nonnull DocumentReference documentReference,
        @Nonnull String field,
        @Nullable Object value,
        Object... moreFieldsAndValues) {
      return performUpdate(
          documentReference,
          Precondition.exists(true),
          FieldPath.fromDotSeparatedString(field),
          value,
          moreFieldsAndValues);
    }
docs: Updates the fields in the document referred to by this DocumentReference. If the document
  doesn't exist yet, the update will fail.
  
  @param documentReference The DocumentReference to update.
  @param field The first field to set.
  @param value The first value to set.
  @param moreFieldsAndValues String and Object pairs with more fields to be set.
  @return The instance for chaining.