Write docs for unit API module

src/ifcopenshell-python/ifcopenshell/api/unit/unassign_unit.py

@@ -18,11 +18,31 @@
 
 
 class Usecase:
-    def __init__(self, file, **settings):
+    def __init__(self, file, units=None):
+        """Unassigns units as default units for the project
+
+        :param units: A list of units to assign as project defaults.
+        :type units: list[ifcopenshell.entity_instance.entity_instance],optional
+        :return: None
+        :rtype: None
+
+        Example::
+
+            # You need a project before you can assign units.
+            ifcopenshell.api.run("root.create_entity", model, ifc_class="IfcProject")
+
+            # Millimeters and square meters
+            length = ifcopenshell.api.run("unit.add_si_unit", model, unit_type="LENGTHUNIT", prefix="MILLI")
+            area = ifcopenshell.api.run("unit.add_si_unit", model, unit_type="AREAUNIT")
+
+            # Make it our default units, if we are doing a metric building
+            ifcopenshell.api.run("unit.assign_unit", model, units=[length, area])
+
+            # Actually, we don't need areas.
+            ifcopenshell.api.run("unit.unassign_unit", model, units=[area])
+        """
         self.file = file
-        self.settings = {"units": None}
-        for key, value in settings.items():
-            self.settings[key] = value
+        self.settings = {"units": units}
 
     def execute(self):
         unit_assignment = self.file.by_type("IfcUnitAssignment")