PendingReleaseNotes: add note on last-seen command

Signed-off-by: Patrick Donnelly <[email protected]>

Author: batrick

PendingReleaseNotes

@@ -37,6 +37,8 @@
   a large buildup of session metadata resulting in the MDS going read-only due to
   the RADOS operation exceeding the size threshold. `mds_session_metadata_threshold`
   config controls the maximum size that a (encoded) session metadata can grow.
+* CephFS: A new "mds last-seen" command is available for querying the last time
+  an MDS was in the FSMap, subject to a pruning threshold.
 * CephFS: For clusters with multiple CephFS file systems, all the snap-schedule
   commands now expect the '--fs' argument.
 * CephFS: The period specifier ``m`` now implies minutes and the period specifier