Write documentation for root API module

src/ifcopenshell-python/ifcopenshell/api/root/reassign_class.py

@@ -21,15 +21,43 @@ import ifcopenshell.util.schema
 
 
 class Usecase:
-    def __init__(self, file, **settings):
+    def __init__(self, file, product=None, ifc_class="IfcBuildingElementProxy", predefined_type=None):
+        """Changes the class of a product
+
+        If you ever created a wall then realised it's meant to be something
+        else, this function lets you change the IFC class whilst retaining all
+        other geometry and relationships.
+
+        This is especially useful when dealing with poorly classified data from
+        proprietary software with limited IFC capabilities.
+
+        :param product: The IfcProduct that you want to change the class of.
+        :type product: ifcopenshell.entity_instance.entity_instance
+        :param ifc_class: The new IFC class you want to change it to.
+        :type ifc_class: str,optional
+        :param predefined_type: In case you want to change the predefined type
+            too. User defined types are also allowed, just type what you want.
+        :type predefined_type: str,optional
+        :return: The newly modified product.
+        :rtype: ifcopenshell.entity_instance.entity_instance
+
+        Example::
+
+            # We have a wall.
+            wall = ifcopenshell.api.run("root.create_entity", model, ifc_class="IfcWall")
+
+            # Oh, did I say wall? I meant slab.
+            slab = ifcopenshell.api.run("root.reassign_class", model, product=wall, ifc_class="IfcSlab")
+
+            # Warning: this will crash since wall doesn't exist any more.
+            print(wall) # Kaboom.
+        """
         self.file = file
         self.settings = {
-            "product": None,
-            "ifc_class": "IfcBuildingElementProxy",
-            "predefined_type": None,
+            "product": product,
+            "ifc_class": ifc_class,
+            "predefined_type": predefined_type,
         }
-        for key, value in settings.items():
-            self.settings[key] = value
 
     def execute(self):
         element = ifcopenshell.util.schema.reassign_class(