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
Moar Java 6 docs here. Just a heads up. If you're going to change the links above, you might want to point to Java 8 here too.