Ahodges/documentation/doc764

@WatsonEmily , please distribute for PM review for usability by our audience.
@stroilova , @shnayder , @rocha  please review for accuracy, completeness, and level of detail, particualrly wrt data czars who will need to revise scripts for post-download processing.
@mhoeber , @srpearce please copy edit.
do we need more detail here?  Does piece of content == a unit? Are there rows for units student skipped?