Course re-run section and refs to it in others

@explorerleslie, @talbs please check for accuracy and completeness
@WatsonEmily please circulate to PMs for level of detail, other likely questions to address, format
@mhoeber , @srpearce please consider this a developmental/copy edit
@sarina, @nasthagiri if you have time I value your input also!

Could we be more specific here? The only user who has access to a course re-run at time of creation is the user who created the course, who has admin access. The "your" tripped me up because it doesn't automatically assume that the person reading this is actually the course re-run creator.