Document ast node types

## Why
Need a cononical source for what nodes are accepted by the generator as well as what node shapes are generated by the core parser. The compiler is extendable but without node shapes it's more difficult to understand what is going on.

## What
Document node shapes or add flow types and coument those.

The issue does not name specific files; start by locating the generator and core parser entry points. Inventory the node shapes accepted by the generator and produced by the parser, then document them or add Flow types and documentation. Done means there is a canonical reference for both sets of node shapes.

Written by the indexing model from the issue text.