Ahodges/documentation/doc764

@WatsonEmily , please distribute for PM review for usability by our audience.
@jhelbert , @stroilova , @shnayder , @rocha  please review for accuracy, completeness, and level of detail, particularly wrt data czars who will need to revise scripts for post-download processing.
@mhoeber , @srpearce please copy edit.
@mhoeber . @srpearce there is a lengthy (and not very clear imo) description of modules in the referenced chapter. If I use "module" here, that will match the term used later in the guide, but may not provide any more value than "piece" or "item" or ...