Clarify lr scheduler frequency (#9843)

pytorch_lightning/core/lightning.py

@@ -1264,6 +1264,9 @@ class LightningModule(
                     "lr_scheduler": {
                         "scheduler": ReduceLROnPlateau(optimizer, ...),
                         "monitor": "metric_to_track",
+                        "frequency": "indicates how often the metric is updated"
+                        # If "monitor" references validation metrics, then "frequency" should be set to a
+                        # multiple of "trainer.check_val_every_n_epoch".
                     },
                 }