Make `vpc_security_group` docs more accurate

Without the `subnet` parameter supplied there's an error `msg: Parameter vpc_security_groups invalid for create command`. (This might be a bug?)

If the VPC security group name rather than ID is supplied there's an error: `msg: Invalid security group , groupId= <some group name>, groupName=.` (Accepting a group name might be a feature enhancement.)

In my case I set the subnet as `default` and used `register` to get the result of the security group creation section and just referred to its `group_id` property.