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.
This is a bit technical, and depends greatly on the tools used. I think we can mention that the files can be streamed, but the explanation of how to do that is out of the scope IMHO.