Add documentation for cost module

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

@@ -18,11 +18,32 @@
 
 
 class Usecase:
-    def __init__(self, file, **settings):
+    def __init__(self, file, cost_item=None, physical_quantity=None):
+        """Removes a quantity assigned to a cost item
+
+        If the quantity is part of a product (e.g. wall), then the quantity will
+        still exist and merely the relationship to the cost item will be
+        removed.
+
+        :param cost_item: The IfcCostItem that the quantity is assigned to
+        :type cost_item: ifcopenshell.entity_instance.entity_instance
+        :param physical_quantity: The IfcPhysicalQuantity to remove
+        :type physical_quantity: ifcopenshell.entity_instance.entity_instance
+        :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)
+            quantity = ifcopenshell.api.run("cost.add_cost_item_quantity", model,
+                cost_item=item, ifc_class="IfcQuantityVolume")
+            # Let's change our mind and delete it
+            ifcopenshell.api.run("cost.remove_cost_item", model,
+                cost_item=item, physical_quantity=quantity)
+        """
         self.file = file
-        self.settings = {"cost_item": None, "physical_quantity": None}
-        for key, value in settings.items():
-            self.settings[key] = value
+        self.settings = {"cost_item": cost_item, "physical_quantity": physical_quantity}
 
     def execute(self):
         if len(self.file.get_inverse(self.settings["physical_quantity"])) == 1: