code: @Nonnull
    public T update(
        @Nonnull DocumentReference documentReference,
        @Nonnull Precondition options,
        @Nonnull String field,
        @Nullable Object value,
        Object... moreFieldsAndValues) {
      Preconditions.checkArgument(
          !options.hasExists(), "Precondition 'exists' cannot be specified for update() calls.");
      return performUpdate(
          documentReference,
          options,
          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 options Preconditions to enforce on this 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.