Add server event command documentation for compute API 2.21

The 2.21 compute API microversion allows listing instance
action events and getting action event details for a deleted
server (which can be useful for auditing until the deleted
server is purged). As far as OSC is concerned it's just a
matter of specifying --os-compute-api-version 2.21 or higher
when listing events or showing event details, so this change
mentions 2.21 in the description of those commands.

Related to nova blueprint os-instance-actions-read-deleted-instances

Change-Id: If276c794f448b6fa5b0845499f3507a159acab85

Author: mriedem

openstackclient/compute/v2/server_event.py

@@ -28,7 +28,10 @@
 
 
 class ListServerEvent(command.Lister):
-    _description = _("List recent events of a server")
+    _description = _(
+        "List recent events of a server. "
+        "Specify ``--os-compute-api-version 2.21`` "
+        "or higher to show events for a deleted server.")
 
     def get_parser(self, prog_name):
         parser = super(ListServerEvent, self).get_parser(prog_name)
@@ -92,8 +95,11 @@ def take_action(self, parsed_args):
 
 class ShowServerEvent(command.ShowOne):
     _description = _(
-        "Show server event details. Specify ``--os-compute-api-version 2.51`` "
-        "or higher to show events for non-admin users.")
+        "Show server event details. "
+        "Specify ``--os-compute-api-version 2.21`` "
+        "or higher to show event details for a deleted server. "
+        "Specify ``--os-compute-api-version 2.51`` "
+        "or higher to show event details for non-admin users.")
 
     def get_parser(self, prog_name):
         parser = super(ShowServerEvent, self).get_parser(prog_name)