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
I'd advocate linking to the most up-to-date docs out there. The only reason I suggest Java 7 is because I assume that most people are probably running Java 7 (I have absolutely no validation there, just a hunch). But seeing as we're likely not to update these links again for a while, linking to Java 8 might be best assuming there's not some huge discrepancy between Java versions + `ExecutorService`s.  So yes, +1 on the Java 8 docs.