Implements ReactiveSortingRepository on SimpleR2dbcRepository. Also changed R2dbcRepository to extend ReactiveSortingRepository and updated comments where it felt reasonable.
Added a single unit test for the new method, and changed the base interface of LegoSetRepository in AbstractR2dbcRepositoryIntegrationTests for integration testing purposes.
Clarify documentation on reactive repository base interfaces
Adds some language calling out ReactiveSortingRepository and fixes consistency between related examples.
Original pull request: #408.