Add documentation comments to config.yml nodes, fields, and enums

Add `comment:` fields to all 77 AST node types, their fields, and
all 6 enums in config.yml, following the same documentation pattern
used by Prism. Each comment includes a prose description and an RBS
code example with `^^^^` location markers showing the source range
covered by the node or field.

The `comment:` keys are ignored by both the Ruby template engine
(templates/template.rb) and the Rust serde deserializer (build.rs),
so this change has no effect on code generation.

Author: Morriar