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.

从 issue 中链接的“Variables from standard input”部分开始。使用提供的 JSON 示例记录可选的 service 字段，并验证该部分是否准确描述了受支持的变量字段。

由索引模型根据 Issue 内容生成。