doc: add note on concurrency of rclone purge

Author: zackees

cmd/purge/purge.go

@@ -22,6 +22,9 @@ include/exclude filters - everything will be removed.  Use the
 delete files. To delete empty directories only, use command
 [rmdir](/commands/rclone_rmdir/) or [rmdirs](/commands/rclone_rmdirs/).
 
+The concurrency of this operation is controlled by the ` + "`--checkers`" + ` global flag. However, some backends will
+implement this command directly, in which case ` + "`--checkers`" + ` will be ignored.
+
 **Important**: Since this can cause data loss, test first with the
 ` + "`--dry-run` or the `--interactive`/`-i`" + ` flag.
 `,