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 and @rocha , does "prod" = edx.org in this naming convention? and, does "edge" = edge.edx.org? (I've never seen them, so don't know)