Add documentation for cost module

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

@@ -18,11 +18,35 @@
 
 
 class Usecase:
-    def __init__(self, file, **settings):
+    def __init__(self, file, physical_quantity=None, attributes=None):
+        """Edits the attributes of an IfcPhysicalQuantity
+
+        For more information about the attributes and data types of an
+        IfcPhysicalQuantity, consult the IFC documentation.
+
+        :param physical_quantity: The IfcPhysicalQuantity entity you want to edit
+        :type physical_quantity: ifcopenshell.entity_instance.entity_instance
+        :param attributes: a dictionary of attribute names and values.
+        :type attributes: dict, optional
+        :return: None
+        :rtype: None
+
+        Example::
+
+            schedule = ifcopenshell.api.run("cost.add_cost_schedule", model)
+            item = ifcopenshell.api.run("cost.add_cost_item", model, cost_schedule=schedule)
+
+            # This cost item will have a unit cost of 5 and a volume of 3
+            value = ifcopenshell.api.run("cost.add_cost_value", model, parent=item)
+            ifcopenshell.api.run("cost.edit_cost_value", model, cost_value=value,
+                attributes={"AppliedValue": 5.0})
+            quantity = ifcopenshell.api.run("cost.add_cost_item_quantity", model,
+                cost_item=item, ifc_class="IfcQuantityVolume")
+            ifcopenshell.api.run("cost.edit_cost_item_quantity", model,
+                physical_quantity=quantity, "attributes": {"VolumeValue": 3.0})
+        """
         self.file = file
-        self.settings = {"physical_quantity": None, "attributes": {}}
-        for key, value in settings.items():
-            self.settings[key] = value
+        self.settings = {"physical_quantity": physical_quantity, "attributes": attributes or {}}
 
     def execute(self):
         for name, value in self.settings["attributes"].items():