Add API documentation for the material module

src/ifcopenshell-python/ifcopenshell/api/material/unassign_material.py

@@ -21,11 +21,36 @@ import ifcopenshell.util.element
 
 
 class Usecase:
-    def __init__(self, file, **settings):
+    def __init__(self, file, product=None):
+        """Removes any material relationship with a product
+
+        A product can only have one material assigned to it, which is why it is
+        not necessary to specify the material to unassign. The material is not
+        removed, only the relationship is removed.
+
+        If the product does not have a material, nothing happens.
+
+        :param product: The IfcProduct that may or may not have a material
+        :type product: ifcopenshell.entity_instance.entity_instance
+        :return: None
+        :rtype: None
+
+        Example::
+
+            concrete = ifcopenshell.api.run("material.add_material", model, name="CON01", category="concrete")
+
+            # Let's imagine a concrete bench made out of concrete.
+            bench_type = ifcopenshell.api.run("root.create_entity", model, ifc_class="IfcFurnitureType")
+            ifcopenshell.api.run("material.assign_material", model,
+                product=bench_type, type="IfcMaterial", material=concrete)
+
+            # Let's change our mind and remove the concrete assignment. The
+            # concrete material still exists, but the bench is no longer made
+            # out of concrete now.
+            ifcopenshell.api.run("material.unassign_material", model, product=bench_type)
+        """
         self.file = file
-        self.settings = {"product": None}
-        for key, value in settings.items():
-            self.settings[key] = value
+        self.settings = {"product": product}
 
     def execute(self):
         if self.settings["product"].is_a("IfcTypeObject"):