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
`ExecutorService` is imported from `java.util.concurrent` whereas `ForkJoinPool` is a port. That's why I suggest to link to two different sources.