Mention API behavior change when over quota limit

[Zhenyu Zheng]

In https://blueprints.launchpad.net/nova/+spec/cells-count-resources-to-check-quota-in-api
we introduced a new workflow of Quota checks. It is possible that concurrent requests can
pass API layer checks, but blocked by conductor layer checks.

This can actually trigger user-noticeable API behavior changes:
As an user, previously, If my request is blocked by quota checks, I will get HTTP 403 response, and no instance records will be left.

After the above mentioned change, it is possible that when my requests failed at conductor layer Quota check and I got an instance in ERROR state. And in an busy cloud, users may
got a lot of ERROR instances according to this and the instance number may beyond the limit.

We should at least mention this behavior change in the release note.

[Takashi Natsume (natsume-takashi)]

Set the status to "In Progress" because this report has an assignee.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/506544

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: stable/pike
Review: https://review.openstack.org/507769

[OpenStack Infra (hudson-openstack)]

Change abandoned by Zhenyu Zheng (<email address hidden>) on branch: master
Review: https://review.openstack.org/506544
Reason: retarget to stable/pike in https://review.openstack.org/#/c/507769/

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/515631

[OpenStack Infra (hudson-openstack)]

Change abandoned by Zhenyu Zheng (<email address hidden>) on branch: stable/pike
Review: https://review.openstack.org/507769
Reason: replaced by https://review.openstack.org/#/c/515631/

[jichenjc (jichenjc)]

should we consider something like a periodic task to clean up those error instances caused by special condition? I know this is a doc change bug but want to consider some way to looks like previous method