Clean up from review comments #3650

	> **CODE REVIEW**
	>
	> *: There should be a test -- probably with multiple-job clusters.

Not above is not done yet

The following are done:
	>
	> *: Job::Hold() and Job::Release() should have doc.
	>
	> *: Job::_onHold should have doc.
	>
	> *: Just pass event->proc to Job::Hold and Job::Release() instead of *event
	> (makes it clearer that proc is the only thing we care about inside
	> those methods).
	>
	> *: I don't think Job::Hold() can ever be called with event == NULL --
	> change if to assert?  (If we continue to pass *event to these methods.)
	>
	> *: Shouldn't Job::TerminateFailure() also free the _onHold memory?
	>
	> *: Why not increment Job::_timesHeld inside Job::Hold() instead of in
	> Dag::ProcessHeldEvent()?
	>
	> *: Print warnings for things like getting a release event for a job
	> proc that was never held or is not currently on hold.