api-ref: Improve the description in unshleve server API

[Brin Zhang]

Since microversion 2.77, allowed request body schema are {'unshelve': null} or {'unshelve': {'availability_zone': <string>}}, and {'unshelve': {}} request body is not allowed.

The reason that not allowed to request {}: {} means allowed everything or any unknown request body which is later ignored so confusing for users. where {'unshelve': null} means allow only empty request body, like for {}. i can pass {'unshelve': {'unknown': 'anything', ....}}, that's not security.

api-ref: https://docs.openstack.org/api-ref/compute/?expanded=unshelve-restore-shelved-server-unshelve-action-detail,show-console-output-os-getconsoleoutput-action-detail#unshelve-restore-shelved-server-unshelve-action

[OpenStack Infra (hudson-openstack)]

This issue was fixed in the openstack/nova 23.0.0.0rc1 release candidate.