Document response fields via data model attributes

[JWCook]

The data models (#145) and related features (#160 #174 #162) are mostly complete, but they could use more documentation:

• Model attributes - these descriptions will then show up in the Sphinx docs
  • With Sphinx we can do 'doc comments' (#: comment above or inline)
  • Adding descriptions to attr.field metadata might be better, though
• For each model, document all the API endpoints it can handle
• more examples

[JWCook]

This is mostly done, but still a work in progress