Doc defensive coding needed by pin payments

[skip ci]

app/services/checkout/form_data_adapter.rb

@@ -33,6 +33,11 @@ module Checkout
       @params[:order][:payments_attributes].first[:source_attributes] = payment_source_params
     end
 
+    # Ensures cc_type is always passed to the model by inferring the type when
+    # the frontend didn't provide it. This fixes Pin Payments specifically
+    # although it might be useful for future payment gateways.
+    #
+    # More details:  app/assets/javascripts/darkswarm/services/checkout.js.coffee#L70-L98
     def fill_in_card_type
       return unless payment_source_attributes