Ensure the gceZone's behavior matches the description in AlloyDBInstance

[maqiuyujoyce]

Checklist

• I did not find a related open enhancement request.
• I understand that enhancement requests filed in the GitHub repository are by default low priority.
• If this request is time-sensitive, I have submitted a corresponding issue with GCP support.

Describe the feature or resource

According to the field description of gceZone in AlloyDB REST API: https://cloud.google.com/alloydb/docs/reference/rest/v1beta/projects.locations.clusters.instances

The Compute Engine zone that the instance should serve from, per https://cloud.google.com/compute/docs/regions-zones 
This can ONLY be specified for ZONAL instances. If present for a REGIONAL instance, an error will be thrown. If this is absent 
for a ZONAL instance, instance is created in a random zone with available capacity.

I tried to create a REGIONAL instance with gceZone specified and the request didn't throw an error: the instance was created successfully.

We should figure out the expected behavior/field description and improve the controller logic/doc accordingly.

Additional information

No response

Importance

No response

[maqiuyujoyce]

@GauravJain21 do you have any suggestions about this API behavior?