Add doc to methods and classes [skip ci]

app/controllers/checkout_controller.rb

@@ -60,6 +60,9 @@ class CheckoutController < Spree::CheckoutController
     end
   end
 
+  # Clears the cached order. Required for #current_order to return a new order
+  # to serve as cart. See https://github.com/spree/spree/blob/1-3-stable/core/lib/spree/core/controller_helpers/order.rb#L14
+  # for details.
   def expire_current_order
     session[:order_id] = nil
     @current_order = nil

app/controllers/spree/paypal_controller_decorator.rb

@@ -7,6 +7,9 @@ Spree::PaypalController.class_eval do
     redirect_to main_app.checkout_path
   end
 
+  # Clears the cached order. Required for #current_order to return a new order
+  # to serve as cart. See https://github.com/spree/spree/blob/1-3-stable/core/lib/spree/core/controller_helpers/order.rb#L14
+  # for details.
   def expire_current_order
     session[:order_id] = nil
     @current_order = nil

app/services/reset_order_service.rb

@@ -1,10 +1,18 @@
+# Builds a new order based on the one specified. This implements the "continue
+# shopping" feature once an order is completed.
 class ResetOrderService
+
+  # Constructor
+  #
+  # @param controller [#expire_current_order, #current_order]
+  # @param order [Spree::Order]
   def initialize(controller, order)
     @controller = controller
     @distributor = order.distributor
     @token = order.token
   end
 
+  # Expires the order currently in use and builds a new one based on it
   def call
     controller.expire_current_order
     build_new_order
@@ -14,6 +22,7 @@ class ResetOrderService
 
   attr_reader :controller, :distributor, :token
 
+  # Builds an order setting the token and distributor of the one specified
   def build_new_order
     new_order = controller.current_order(true)
     new_order.set_distributor!(distributor)