code: @Nonnull
    public CollectionReference collection(@Nonnull String collectionPath) {
      return new CollectionReference(firestore, path.append(collectionPath));
    }
docs: Gets a CollectionReference instance that refers to the collection that is a child of this
  document.
  
  @param collectionPath A relative and slash-separated path to a collection.
  @return The CollectionReference instance.