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.
@mulby I actually didn't do that deliberately. I set up the separate section on the AWS buckets, and the two bucket-navigation procedural steps, to deal with the concept of getting the files from different buckets. I thought it might overload an inexperienced data czar to get the info about the types of data and the file names AND the buckets all in one section. Let me think a bit.