Write docs for system API module

src/ifcopenshell-python/ifcopenshell/api/system/unassign_port.py

@@ -21,14 +21,38 @@ import ifcopenshell.api
 
 
 class Usecase:
-    def __init__(self, file, **settings):
+    def __init__(self, file, element=None, port=None):
+        """Unassigns a port to an element
+
+        Ports are typically always assigned to a distribution element, but in
+        some edge cases you may want to unassign the port to create an orphaned
+        port for cleaning or patchin purposes.
+
+        :param element: The IfcDistributionElement to unassign the port from.
+        :type element: ifcopenshell.entity_instance.entity_instance
+        :param port: The IfcDistributionPort you want to unassign.
+        :type port: ifcopenshell.entity_instance.entity_instance
+        :return: None
+        :rtype: None
+
+        Example::
+
+            # Create a duct
+            duct = ifcopenshell.api.run("root.create_entity", model,
+                ifc_class="IfcDuctSegment", predefined_type="RIGIDSEGMENT")
+
+            # Create 2 ports, one for either end.
+            port1 = ifcopenshell.api.run("system.add_port", model, element=duct)
+            port2 = ifcopenshell.api.run("system.add_port", model, element=duct)
+
+            # Unassign one port for some weird reason.
+            ifcopenshell.api.run("system.unassign_port", model, element=duct, port=port1)
+        """
         self.file = file
         self.settings = {
-            "element": None,
-            "port": None,
+            "element": element,
+            "port": port,
         }
-        for key, value in settings.items():
-            self.settings[key] = value
 
     def execute(self):
         if self.file.schema == "IFC2X3":