docs(backup): add information about currently supported events (#21987)

Some of the listed events are no longer supported and don't do anything. Note this fact and refer readers to a page in the AWS Backup guide that lists the currently-supported events.

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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: Jerry Kindall

packages/@aws-cdk/aws-backup/lib/vault.ts

@@ -103,7 +103,10 @@ export interface BackupVaultProps {
 }
 
 /**
- * Backup vault events
+ * Backup vault events. Some events are no longer supported and will not return
+ * statuses or notifications.
+ *
+ * @see https://docs.aws.amazon.com/aws-backup/latest/devguide/API_PutBackupVaultNotifications.html#API_PutBackupVaultNotifications_RequestBody
  */
 export enum BackupVaultEvents {
   /** BACKUP_JOB_STARTED */