fix(complexity_router): hint that the classifier plugin registry loads at startup and reload

An operator whose name is declared but whose proxy has not reloaded otherwise
reads the unknown-name error as a typo hunt

litellm/router_strategy/complexity_router/config.py

@@ -565,7 +565,7 @@ class ComplexityRouterConfig(BaseModel):
         if registered is None:
             raise ValueError(
                 f"{value!r} is not a registered classifier plugin; declare it under "
-                "classifier_plugins in the proxy config"
+                "classifier_plugins in the proxy config (the registry loads at startup and config reload)"
             )
         return registered