Add server event command documentation for compute API 2.21
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

openstack/python-openstackclient,openstack/python-openstackclient,dtroyer/python-openstackclient,dtroyer/python-openstackclient