VMSS commands that accept multiple instances need default text

[JasonRShaver]

For commands like stop that can apply to the scale set or an instance should state that not providing an instance will apply the operation to the entire set.

$ az vm scaleset stop --help

Command
    az vm scaleset stop

Arguments
    --name -n           [Required]
    --resource-group -g [Required]: Name of resource group.
    --instance-ids                : Space separated ids, such as 0 2 3.

Global Arguments
    --debug                       : Increase logging verbosity to show all debug logs.
    --help -h                     : Show this help message and exit.
    --output -o                   : Output format.  Allowed values: json, tsv, list.  Default: json.
    --query                       : JMESPath query string. See http://jmespath.org/ for more
                                    information and examples.
    --verbose                     : Increase logging verbosity. Use --debug for full debug logs.