doc/cephfs: mention need of confirmation for "ceph fs fail"

Signed-off-by: Rishabh Dave <ridave@redhat.com>

Author: rishabh-d-dave

doc/cephfs/administration.rst

@@ -193,7 +193,11 @@ file system and MDS daemons down, use the ``ceph fs fail`` command:
 
 ::
 
-    ceph fs fail <fs_name>
+    ceph fs fail <fs_name> {--yes-i-really-mean-it}
+
+.. note:: Note that confirmation flag is optional because it is only required
+   when the MDS is active and has health warning MDS_TRIM or
+   MDS_CACHE_OVERSIZED.
 
 This command sets a file system flag to prevent standbys from
 activating on the file system (the ``joinable`` flag).