@staylor You have to iterate over your list of documents using a for loop. Right now you are defining doc as a list, so essentially in line 18 you are saying param_list = ListObject.FamilyManager.GetParameters() See below example, when I query the object type of the input without a for loop, it shows me the object is a list. If I query the object type of the input elements with the for loop, it returns the object type of the items in the list
So, you need to put your code within a for loop that is iterating over the list of documents, that way the code is running through each input document. Hope this helps you in the right direction 
(Side note, there is a node in the package “DanEDU Dynamo” that does exactly what you are trying to do, so you won’t have to end up editing any Python if you get that package)