doc: update pendingreleasenotes for rbd qos vac feature

Signed-off-by: Yite Gu <guyite@bytedance.com>

Author: YiteGu

PendingReleaseNotes.md

@@ -6,5 +6,6 @@
 
 - nfs: allow changing NFS-server through ControllerModifyVolume [PR](https://github.com/ceph/ceph-csi/pull/5829)
 - util: add support for GKLM KMS over KMIP [PR](https://github.com/ceph/ceph-csi/pull/6048)
+- rbd: use volumeattributesclass feature implement rbd volume qos [PR](https://github.com/ceph/ceph-csi/pull/6160)
 
 ## NOTE