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
Scala 2.11.x does not require Java 7; that's why I didn't want to link to Java 7 docs. However, if it's OK to link to Java 7 docs, we might as well link to Java 8 docs. What do people think?