[#73] Documentation

Per https://github.com/whitequark/parser/issues/73#issuecomment-20466607, documenting the public api (and marking the rest as private or internal, as appropriate)

- [x] parser.rb
- [x] parser/base.rb
- [x] diagnostic.rb
- [x] rewriter.rb
- [x] syntax_error.rb
- [x] ast/
- [x] diagnostic/
- source/
  - [ ] buffer.rb
  - [ ] comment.rb
  - [ ] map.rb
  - [ ] range.rb
  - [ ] rewriter.rb
  - [ ] commit/
  - [ ] map/
  - [ ] rewriter/