Changes to JEP 453

[Attila Kelemen]

Thanks for the improvements, I think all of them are for the better.
Just a few remarks:

This does not affect the JEP itself, but in
`StructuredTaskScope.SubtaskImpl.get()`, can we have some small
quality of life improvement:

>From this:

```
if (result instanceof AltResult) {
    if (result == RESULT_NULL) return null;
} else if (result != null) {
    @SuppressWarnings("unchecked")
    T r = (T) result;
    return r;
}
throw new IllegalStateException("Task not completed or did not
complete successfully");
```

to this:

```
if (result instanceof AltResult alt) {
    if (result == RESULT_NULL) return null;
    throw new IllegalStateException("Task did not complete
successfully", alt.exception());
} else if (result != null) {
    @SuppressWarnings("unchecked")
    T r = (T) result;
    return r;
}
throw new IllegalStateException("Task not completed");
```

Since the information is known, it would help debugging when people
make a mistake, rather than punishing them by hiding the exception
(even though it is readily available).


> 4. We've added the com.sun.management.Threads.currentThreadEnclosingScopes() method that returns a string with the description of the current structured context -- i.e. the stack trace for the current thread and the enclosing scopes with their owners' respective stack traces -- all the way up the hierarchy.
>

Very minor thing: There is a typo in the javadoc: it says "purpoes"
instead of "purpose".

Attila