api-ref: volume-create request sample is incorrect

[Brian Rosmaita]

After change I0ece6e279048abcc04b3, including the "multiattach" request parameter in a volume-create request should return a 400 (Bad Request) response. The sample request in the api-ref, however, still contains this parameter.

The functional api-ref-samples tests have not caught this because they pass '"multiattach": false':

https://opendev.org/openstack/cinder/src/commit/42ef47ba26ab15c526afa7eec7f243bd1c97f770/cinder/tests/functional/api_sample_tests/samples/volumes/volume-create-request.json.tpl#L7

The test passes because the "multiattach" parameter was not removed from the request schema so that an informative error message could be returned. You can see that a False value of the parameter is silently ignored here:

https://opendev.org/openstack/cinder/src/commit/42ef47ba26ab15c526afa7eec7f243bd1c97f770/cinder/api/v3/volumes.py#L389-L396

The request sample should be corrected, however, because the proper way to request a multiattach volume is to use a volume-type that allows multiattach.