code: @Nonnull
    public T update(
        @Nonnull DocumentReference documentReference, @Nonnull Map<String, Object> fields) {
      return performUpdate(
          documentReference,
          convertToFieldPaths(fields, /* splitOnDots= */ true),
          Precondition.exists(true));
    }
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.
  @return The instance for chaining.