Document `rubocops/shared` modules.

Library/Homebrew/rubocops/shared/desc_helper.rb

@@ -5,6 +5,8 @@ require "rubocops/shared/helper_functions"
 module RuboCop
   module Cop
     # This module performs common checks the `desc` field in both Formulae and Casks.
+    #
+    # @api private
     module DescHelper
       include HelperFunctions
 

Library/Homebrew/rubocops/shared/helper_functions.rb

@@ -2,6 +2,9 @@
 
 module RuboCop
   module Cop
+    # Helper functions for cops.
+    #
+    # @api private
     module HelperFunctions
       include RangeHelp