SI-8590 Expand doc comments for ExecutionContext

- link to Java API docs for ForkJoinPool-based default implementation
- add example for creating an execution context from a `java.util.concurrent.ExecutorService`
- add tags for parameters and return values
@heathermiller @viktorklang Agreed. Given that we won't be able to resolve the issues of `prepare()` in 2.11.x, would you be OK if I added @mariusaeriksen 's suggested doc comment to `prepare()`?