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!

"The course run date that you enter does not affect the course start date" (or "The date that you enter in the **Course Run** field does not affect the date in the **Course Start Date** field")