Add more information in the release notes about extraFlags

Signed-off-by: Florent Poinsard <[email protected]>

docs/release-notes/2_13_0_summary.md

@@ -7,5 +7,9 @@ implementing this change is available here: [#553](https://github.com/planetscal
 
 This feature is for now experimental as we await feedback from the community on its usage. There are a few things to
 take into account when using this feature:
+
 - If you are using the `xtrabackup` engine, your vttablet pods will need more memory, think about provisioning more memory for it.
-- If you are using the `builtin` engine, you will lose a replica during the backup, think about adding a new tablet.
+- If you are using the `builtin` engine, you will lose a replica during the backup, think about adding a new tablet.
+
+If you are usually using specific flags when taking backups with `vtctldclient` you can set those flags on the `extraFlags`
+field of the backup `strategy` (`VitessBackupScheduleStrategy`).