Add documentation for cost module

src/ifcopenshell-python/ifcopenshell/api/cost/add_cost_item.py

@@ -20,11 +20,36 @@ import ifcopenshell.api
 
 
 class Usecase:
-    def __init__(self, file, **settings):
+    def __init__(self, file, cost_schedule=None, cost_item=None):
+        """Add a new cost item
+
+        A cost item represents a single line item in a cost schedule. Cost items
+        may then be broken down into cost subitems.
+
+        :param cost_schedule: If the cost item is to be added as a root or top
+            level cost item to a cost schedule, the IfcCostSchedule may be
+            specified. This is mutually exlclusive to the cost_item parameter.
+        :type cost_schedule: ifcopenshell.entity_instance.entity_instance
+        :param cost_item: If the cost item is to be added as a subitem to an
+            existing cost item, the parent IfcCostItem may be specified. This is
+            mutually exclusive to the cost_schedule parameter.
+        :type cost_item: ifcopenshell.entity_instance.entity_instance
+        :return: The newly created IfcCostItem
+        :rtype: ifcopenshell.entity_instance.entity_instance
+
+        Example::
+
+            # The very first cost item must be in a cost schedule
+            schedule = ifcopenshell.api.run("cost.add_cost_schedule", model)
+
+            # You may add cost items as top level item in the schedule
+            item1 = ifcopenshell.api.run("cost.add_cost_item", model, cost_schedule=schedule)
+
+            # Alternatively you may add them as subitems
+            item2 = ifcopenshell.api.run("cost.add_cost_item", model, cost_item=item1)
+        """
         self.file = file
-        self.settings = {"cost_schedule": None, "cost_item": None}
-        for key, value in settings.items():
-            self.settings[key] = value
+        self.settings = {"cost_schedule": cost_schedule, "cost_item": cost_item}
 
     def execute(self):
         cost_item = ifcopenshell.api.run("root.create_entity", self.file, ifc_class="IfcCostItem")