Update start-backup.md

docs/usage/start-backup.md

@@ -38,7 +38,11 @@
      ```
 
      During a *physical* backup, Percona Backup for MongoDB  copies the contents of the `dbpath` directory (data and metadata files, indexes, journal and logs) from every shard and config server replica set to the backup storage. 
+     
+    !!! warning 
 
+        During the period the backup cursor is open, database checkpoints can be created, but no checkpoints can be deleted. This may result in significant file growth.
+
      Starting with [2.4.0](../release-notes/2.4.0.md), PBM doesn't stop [point-in-time recovery oplog slicing](../features/point-in-time-recovery.md#oplog-slicing), if it's enabled, but runs it in parallel. This ensures [point-in-time recovery](pitr-tutorial.md) to any timestamp if it takes too long (e.g. hours) to make a backup snapshot.
 
 === ":material-select-multiple: Selective"