Add API docs for constraint module

src/ifcopenshell-python/ifcopenshell/api/constraint/add_metric.py

@@ -20,13 +20,29 @@ import ifcopenshell
 
 
 class Usecase:
-    def __init__(self, file, **settings):
+    def __init__(self, file, objective=None):
+        """Add a new metric benchmark
+
+        Qualitative constraints may have a series of quantitative benchmarks
+        linked to it known as metrics. Metrics may be parametrically linked to
+        computed model properties or quantities. Metrics need to be satisfied
+        to meet the objective of the constraint.
+
+        :param objective: The IfcObjective that this metric is a benchmark of.
+        :type objective: ifcopenshell.entity_instance.entity_instance
+        :return: The newly created IfcMetric entity
+        :rtype: ifcopenshell.entity_instance.entity_instance
+
+        Example::
+
+            objective = ifcopenshell.api.run("constraint.add_objective", model)
+            metric = ifcopenshell.api.run("constraint.add_metric", model,
+                objective=objective)
+        """
         self.file = file
         self.settings = {
-            "objective": None,
+            "objective": objective,
         }
-        for key, value in settings.items():
-            self.settings[key] = value
 
     def execute(self):
         metric = self.file.create_entity(