I will answer my own question as I have just found the answer by myself. The following section in the Spring Data MongoDB documentation lists all supported keywords that are used by Spring for its query derivation:
http://docs.spring.io/spring-data/mongodb/docs/current/reference/html/#repository-query-keywords
The following implementation works for the use case described above:
@Repository
public interface QuestionRepository extends MongoRepository<Question, String> {
List<Question> findByTagsIn(List<String> tags);
}