Add docs for id_param option of load_resource

Added in https://github.com/ryanb/cancan/pull/425 but lacked documentation

lib/cancan/controller_additions.rb

@@ -96,6 +96,11 @@ module CanCan
       #
       #     load_resource :find_by => :permalink # will use find_by_permalink!(params[:id])
       #
+      # [:+id_param+]
+      #   Find using a param key other than :id. For example:
+      #
+      #     load_resource :id_key => :url # will use find(params[:url])
+      #
       # [:+collection+]
       #   Specify which actions are resource collection actions in addition to :+index+. This
       #   is usually not necessary because it will try to guess depending on if the id param is present.