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
- expand doc comment for `prepare`
Merge pull request #3874 from phaller/issue/8590

SI-8590 Expand doc comments for ExecutionContext