code: func (c *Collection) UpsertId(id interface{}, update interface{}) (info *ChangeInfo, err error) {
  	return c.Upsert(bson.D{{"_id", id}}, update)
  }
docs: // UpsertId is a convenience helper equivalent to:
  //
  //     info, err := collection.Upsert(bson.M{"_id": id}, update)
  //
  // See the Upsert method for more details.