Add server add/remove volume description for microversion 2.20

The compute API 2.20 microversion allows attaching and detaching
a volume to/from a server with status SHELVED or SHELVED_OFFLOADED.
For OSC this just means the user has to specify the appropriate
minimum microversion to make that work, so this change mentions
that in the "server add volume" and "server remove volume" command
description.

Related to nova blueprint volume-ops-when-shelved

Change-Id: I4824175e5d9e124e3bd9e9a8fd5a89277efc6cff

Author: mriedem

openstackclient/compute/v2/server.py

@@ -424,7 +424,10 @@ def take_action(self, parsed_args):
 
 
 class AddServerVolume(command.Command):
-    _description = _("Add volume to server")
+    _description = _(
+        "Add volume to server. "
+        "Specify ``--os-compute-api-version 2.20`` or higher to add a volume "
+        "to a server with status ``SHELVED`` or ``SHELVED_OFFLOADED``.")
 
     def get_parser(self, prog_name):
         parser = super(AddServerVolume, self).get_parser(prog_name)
@@ -1949,7 +1952,11 @@ def take_action(self, parsed_args):
 
 
 class RemoveServerVolume(command.Command):
-    _description = _("Remove volume from server")
+    _description = _(
+        "Remove volume from server. "
+        "Specify ``--os-compute-api-version 2.20`` or higher to remove a "
+        "volume from a server with status ``SHELVED`` or "
+        "``SHELVED_OFFLOADED``.")
 
     def get_parser(self, prog_name):
         parser = super(RemoveServerVolume, self).get_parser(prog_name)