code: public BsonDocument toUpdateDocument() {
      final List<BsonElement> unsets = new ArrayList<>();
      for (final String removedField : this.removedFields) {
        unsets.add(new BsonElement(removedField, new BsonBoolean(true)));
      }
      final BsonDocument updateDocument = new BsonDocument();
  
      if (this.updatedFields.size() > 0) {
        updateDocument.append("$set", this.updatedFields);
      }
  
      if (unsets.size() > 0) {
        updateDocument.append("$unset", new BsonDocument(unsets));
      }
  
      return updateDocument;
    }
docs: Convert this update description to an update document.
  
  @return an update document with the appropriate $set and $unset documents.