code: @Nonnull
    public DocumentReference document(@Nonnull String childPath) {
      ResourcePath documentPath = path.append(childPath);
      Preconditions.checkArgument(
          documentPath.isDocument(),
          String.format("Path should point to a Document Reference: %s", path));
      return new DocumentReference(firestore, documentPath);
    }
docs: Gets a DocumentReference instance that refers to the document that is a child of this
  Collection.
  
  @param childPath A relative and slash-separated path to a document.
  @return The DocumentReference instance.