Document `Cask::DSL`.

Library/Homebrew/cask/dsl.rb

Library/Homebrew/cask/dsl/appcast.rb

Library/Homebrew/cask/dsl/base.rb

Library/Homebrew/cask/dsl/caveats.rb

Library/Homebrew/cask/dsl/conflicts_with.rb

Library/Homebrew/cask/dsl/container.rb

Library/Homebrew/cask/dsl/depends_on.rb

Library/Homebrew/cask/dsl/postflight.rb

Library/Homebrew/cask/dsl/preflight.rb

Library/Homebrew/cask/dsl/uninstall_postflight.rb

Library/Homebrew/cask/dsl/uninstall_preflight.rb

Library/Homebrew/cask/dsl/version.rb