triage: document model diagnosis toggle

ariadne/services/testing_triage_diagnosis.py

@@ -41,6 +41,8 @@ def latest_testing_triage_diagnosis(storage: Storage) -> dict[str, Any] | None:
 
 
 def model_diagnosis_enabled() -> bool:
+    """Return whether Ariadne has a local testing diagnosis model configured."""
+
     return bool(_model_url())