api-ref: document ordering for instance actions and events
api-ref: document ordering for instance actions and events

Whenever I use the os-instance-actions API I have to look at
the DB API source code to figure out the sort order of the
resulting instanceActions and each action's events, which
is desc(created_at) in both cases (and desc(id) but that should
not matter here since the id is not exposed in the API).

This change mentions the resulting sort order of those fields
in the API reference so I can stop looking at source code.

[1] https://github.com/openstack/nova/blob/e7ae6c65c/nova/db/sqlalchemy/api.py#L5149
[2] https://github.com/openstack/nova/blob/e7ae6c65c/nova/db/sqlalchemy/api.py#L5289

Change-Id: Ib5758bc21296e8b6c041198661c147b8e99d57e5

rahulunair/nova,mahak/nova,klmitch/nova,openstack/nova,klmitch/nova,mahak/nova,openstack/nova,mahak/nova,rahulunair/nova,klmitch/nova,openstack/nova,rahulunair/nova,klmitch/nova