Add a notice for pd configuration (pingcap#18897) (pingcap#19012)

pd-configuration-file.md

@@ -204,6 +204,13 @@ Configuration items related to monitoring
 
 Configuration items related to scheduling
 
+> **Note:**
+>
+> To modify these PD configuration items related to `schedule`, choose one of the following methods based on your cluster status:
+>
+> - For clusters to be newly deployed, you can modify the PD configuration file directly.
+> - For existing clusters, use the command-line tool [PD Control](/pd-control.md) to make changes instead. Direct modifications to these PD configuration items related to `schedule` in the configuration file do not take effect on existing clusters.
+
 ### `max-merge-region-size`
 
 + Controls the size limit of `Region Merge`. When the Region size is greater than the specified value, PD does not merge the Region with the adjacent Regions.