Field "service" not documented for variables in https://github.com/adobe/aio-cli-plugin-cloudmanager#variables-from-standard-input

The JSON may also include a service field per variable like this
```
{
"name": "AES_KEY",
"type": "secretString",
"value": "****",
"service": "preview"
}
```
However this is not documented in https://github.com/adobe/aio-cli-plugin-cloudmanager#variables-from-standard-input.

Start with the “Variables from standard input” section linked in the issue. Document the optional service field using the provided JSON example, and verify that the section accurately describes the supported variable fields.

Written by the indexing model from the issue text.