Improve --export option docs.

          @output_dir = path == "-" ? :stdout : canonical(path)

        end

        opts.on('--export=TYPE',

        opts.on('--export=full/examples',

          "Exports docs in JSON.",

          "",

          "TYPE is one of:",

          "For each JavaScript class a JSON file gets written,",

          "the contents of which are as follows:",

          "",

          "- full     - full class docs.",

          "- examples - extracts inline examples from classes.") do |format|

          "- full     - docs and metadata for class and its members.",

          "- examples - inline examples from classes and guides.") do |format|

          export_type = format.to_sym

          if [:full, :examples].include?(export_type)

            @export = export_type