Finish all in-progress documentation

[nikolausWest]

There are several pieces of documentation that are in a half-done state. This issue is a reminder to go through all docs an look for TODOs and empty code blocks before the release.

[teh-cmc]

FYI my plan is to just comment those out for this release -- everything in that page is just a nice bonus.

[abey79]

Not sure if that can be closed, but I removed it from the 0.21 milestone.