code: @Nonnull
    public T update(
        @Nonnull DocumentReference documentReference,
        @Nonnull Map<String, Object> fields,
        Precondition options) {
      Preconditions.checkArgument(
          !options.hasExists(), "Precondition 'exists' cannot be specified for update() calls.");
      return performUpdate(
          documentReference, convertToFieldPaths(fields, /* splitOnDots= */ true), options);
    }
docs: Updates 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 fields A Map containing the fields and values with which to update the document.
  @param options Preconditions to enforce on this update.
  @return The instance for chaining.