FederatedAccounts module requires that FederatedAccountService interface is bound to an existing service, but doesn't bind to any by default. This is so you can easily provide a custom implementation for FederatedAccountService, using your own persistence model. Two implementations are provided by default for FederatedAccountService: DefaultHibernateFederatedAccountServiceImpl and DefaultJpaFederatedAccountServiceImpl.
Improvements to the documentation are more than welcome. However, we've purposefully tried to keep the documentation light since this family of library was purposefully designed to cover a wide range of use cases and documenting all the different options and possibilities would make the documentation too heavy and complex. The underlying libraries are well battle tested and all of the federatedaccount libraries are in use in production applications today. If you are new to the world of Oauth and federated account systems in general, it's best to dive in by integrating with a single provider first and build up from there.