Document `Pkg`.

Library/Homebrew/cask/pkg.rb

@@ -3,6 +3,9 @@
 require "cask/macos"
 
 module Cask
+  # Helper class for uninstalling `.pkg` installers.
+  #
+  # @api private
   class Pkg
     def self.all_matching(regexp, command)
       command.run("/usr/sbin/pkgutil", args: ["--pkgs=#{regexp}"]).stdout.split("\n").map do |package_id|