Document `Artifact`.

Library/Homebrew/cask/artifact.rb

Library/Homebrew/cask/artifact/abstract_artifact.rb

Library/Homebrew/cask/artifact/abstract_flight_block.rb

Library/Homebrew/cask/artifact/abstract_uninstall.rb

Library/Homebrew/cask/artifact/app.rb

Library/Homebrew/cask/artifact/artifact.rb

Library/Homebrew/cask/artifact/audio_unit_plugin.rb

Library/Homebrew/cask/artifact/binary.rb

Library/Homebrew/cask/artifact/colorpicker.rb

Library/Homebrew/cask/artifact/dictionary.rb

Library/Homebrew/cask/artifact/font.rb

Library/Homebrew/cask/artifact/input_method.rb

Library/Homebrew/cask/artifact/installer.rb

Library/Homebrew/cask/artifact/internet_plugin.rb

Library/Homebrew/cask/artifact/manpage.rb

Library/Homebrew/cask/artifact/mdimporter.rb

Library/Homebrew/cask/artifact/moved.rb

Library/Homebrew/cask/artifact/pkg.rb

Library/Homebrew/cask/artifact/postflight_block.rb

Library/Homebrew/cask/artifact/preflight_block.rb