[#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/
  - [x] buffer.rb
  - [x] comment.rb
  - [x] map.rb
  - [x] range.rb
  - [ ] rewriter.rb
  - comment/
    - [x] associator.rb
  - map/
    - [x] collection.rb
    - [x] condition.rb
    - [x] constant.rb
    - [x] definition.rb
    - [x] for.rb
    - [x] keyword.rb
    - [x] operator.rb
    - [x] rescue_body.rb
    - [x] send.rb
    - [x] ternary.rb
    - [x] variable.rb