Add note about microversion 2.87 in server rescue help

The ability to rescue a volume-backed server was added in compute
microversion 2.87 [1].

This adds a note to the command help to improve user experience.

[1] https://docs.openstack.org/nova/latest/user/rescue.html

Change-Id: I5f40c3ca28e13bd1f979bc5f8c337302a3b9a5be

Author: melwitt

openstackclient/compute/v2/server.py

@@ -3645,7 +3645,11 @@ def take_action(self, parsed_args):
 
 
 class RescueServer(command.Command):
-    _description = _("Put server in rescue mode")
+    _description = _(
+        "Put server in rescue mode. "
+        "Specify ``--os-compute-api-version 2.87`` or higher to rescue a "
+        "server booted from a volume."
+    )
 
     def get_parser(self, prog_name):
         parser = super(RescueServer, self).get_parser(prog_name)