docs: DocumentSet remove(ResourcePath key) {
      QueryDocumentSnapshot document = keyIndex.get(key);
      if (document == null) {
        return this;
      }
  
      ImmutableSortedMap<ResourcePath, QueryDocumentSnapshot> newKeyIndex = keyIndex.remove(key);
      ImmutableSortedSet<QueryDocumentSnapshot> newSortedSet = sortedSet.remove(document);
      return new DocumentSet(newKeyIndex, newSortedSet);
    }