PendingReleaseNotes: add note on the client incompatibility health wa…

…rning and feature bit

Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>
(cherry picked from commit e70f005)

PendingReleaseNotes

@@ -115,6 +115,13 @@
    `ceph config get mgr mgr/volumes/snapshot_clone_no_wait`
   and it can be disabled by using:
    `ceph config set mgr mgr/volumes/snapshot_clone_no_wait false`
+* CephFS: fixes to the implementation of the ``root_squash`` mechanism enabled
+  via cephx ``mds`` caps on a client credential require a new client feature
+  bit, ``client_mds_auth_caps``. Clients using credentials with ``root_squash``
+  without this feature will trigger the MDS to raise a HEALTH_ERR on the
+  cluster, MDS_CLIENTS_BROKEN_ROOTSQUASH. See the documentation on this warning
+  and the new feature bit for more information.
+
 
 >=18.0.0