Add comment about APIResource exclusion for clarity

lib/stripe/stripe_object.rb

@@ -324,6 +324,11 @@ module Stripe
       when nil
         ''
 
+      # The logic here is that essentially any object embedded in another
+      # object that had a `type` is actually an API resource of a different
+      # type that's been included in the response. These other resources must
+      # be updated from their proper endpoints, and therefore they are not
+      # included when serializing even if they've been modified.
       when APIResource
         nil