Add documentation for Distinct Values API

[obruchez]

The Distinct Values API has been kept "private" so far, but should probably be documented, as some customers are implementing their own persistence providers, which should then implement the Distinct Values API.

[obruchez]

• Check what would happen (e.g. on the Summary page) if a provider were to not support the Distinct Values API.
• Is it possible to implement some fallback code?
• Would it be possible to use dynamic dropdowns with paging? Should we then add pagination to the Distinct Values API? See calledPagedService as an example of code lazily consuming data until the required results have been produced.