Update man page of gnt-cluster regarding file-storage-dir

This extends the description of 'gnt-cluster init' and
'gnt-cluster modify' with respect to the file storage
directory.
Signed-off-by: Helga Velroyen <helgav@google.com>
Reviewed-by: Thomas Thrainer <thomasth@google.com>

man/gnt-cluster.rst

@@ -271,7 +271,8 @@ use for storing the instance disk files when using file storage as
 backend for instance disks. Note that the file storage dir must be
 an allowed directory for file storage. Those directories are specified
 in the ``@SYSCONFDIR@/ganeti/file-storage-paths`` file. The file storage
-directory can also be a subdirectory of an allowed one.
+directory can also be a subdirectory of an allowed one. The file storage
+directory should be present on all nodes.
 
 The ``--prealloc-wipe-disks`` sets a cluster wide configuration value
 for wiping disks prior to allocation and size changes (``gnt-instance
@@ -631,6 +632,7 @@ MODIFY
 | [\--ipolicy-vcpu-ratio *ratio*]
 | [\--enabled-disk-templates *template* [,*template*...]]
 | [\--drbd-usermode-helper *helper*]
+| [\--file-storage-dir *dir*]
 
 
 Modify the options for the cluster.
@@ -640,8 +642,8 @@ The ``--vg-name``, ``--enabled-hypervisors``, ``-H (--hypervisor-parameters)``,
 ``-C (--candidate-pool-size)``, ``--maintain-node-health``,
 ``--prealloc-wipe-disks``, ``--uid-pool``, ``--node-parameters``,
 ``--master-netdev``, ``--master-netmask``, ``--use-external-mip-script``,
-``--drbd-usermode-helper``, and ``--enabled-disk-templates`` options are
-described in the **init** command.
+``--drbd-usermode-helper``, ``--file-storage-dir``, and
+``--enabled-disk-templates`` options are described in the **init** command.
 
 The ``--hypervisor-state`` and ``--disk-state`` options are described in
 detail in **ganeti**\(7).