code: @Nonnull
    public ApiFuture<WriteResult> update(
        @Nonnull Precondition options,
        @Nonnull String field,
        @Nullable Object value,
        Object... moreFieldsAndValues) {
      WriteBatch writeBatch = firestore.batch();
      return extractFirst(
          writeBatch.update(this, options, field, value, moreFieldsAndValues).commit());
    }
docs: Updates the fields in the document referred to by this DocumentReference. If the document
  doesn't exist yet, the update will fail.
  
  @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 A ApiFuture that will be resolved when the write finishes.