Rollup merge of #23428 - Manishearth:ast-doc, r=huon

 I often have to run `ast-json` or look into the pretty-printer source to figure out what the fields of an AST enum mean. I've tried to document most of what I know (and some semi-obvious stuff).

r? @steveklabnik

f? @eddyb

Author: Manishearth