Revisions to ai grading arch doc

Small clarifications.

* Clarify that "reliability" of the task queue is "at least once" not "at most once".
* Clarify that grading tasks scheduled before training completes will be rescheduled once training completes.
* Add rubric foreign key to the grading task workflow, so we can find which tasks to reschedule when a training task completes. 

@stephensanchez