Added a line of motivation for updating cluster_time

Author: caseyclements

test/unified_format.py

@@ -1880,7 +1880,8 @@ def _run_scenario(self, spec, uri=None):
         # process initialData
         if "initialData" in self.TEST_SPEC:
             self.insert_initial_data(self.TEST_SPEC.get("initialData", []))
-            # advance cluster times of session entities
+            # advance cluster times of session entities,
+            # to ensure consistency in transactions against a sharded deployment,
             cluster_time = self.client.admin.command("ping").get("$clusterTime")
             if cluster_time:
                 for entity in self.entity_map._entities.values():