Add student training assessment information for course authors

@mhoeber @lamagnifica @WatsonEmily Please review as soon as possible - this needs to be finished by Wednesday afternoon (6/4). Thank you!
2nd sentence, it's not clear to me if maybe you mean
You can also include a student training assessment paired with either a peer assessment only or a self assessment only. (?)
Or is that 2nd sentence even needed, since the issue is about sequence rather than selection? Maybe strike that 2nd sentence and start the note with "You can include any combination of the three assessment types in your problem. However, if..."