Clarify details in PipelineExecutionStepState

The `details` element in https://github.com/AdobeDocs/cloudmanager-api-docs/blob/b6c0678a414f112dba41e3bc8ecbf324385cd20f/swagger-specs/api.yaml#L1707 does not define which map keys to expect there.

I have seen the following details map

```
"environmentUrls": [
{
"instanceType": "author",
"instanceUrl": "https://author-p7802-e44859.adobeaemcloud.com"
},
{
"instanceType": "publish",
"instanceUrl": "https://publish-p7802-e44859.adobeaemcloud.com"
}
],
"deploymentStepDescription": "[{\"id\":1,\"stepAction\":\"update-author-indexes\",\"updated\":\"2020-11-02T13:24:11.870+0000\"},{\"id\":2,\"stepAction\":\"update-publish-indexes\",\"updated\":\"2020-11-02T13:27:22.581+0000\"},{\"id\":3,\"stepAction\":\"update-services\",\"updated\":\"2020-11-02T13:27:46.629+0000\"},{\"id\":4,\"stepAction\":\"install-mutable-content\",\"updated\":\"2020-11-02T13:37:18.741+0000\"}]"
```

Would be nice to clarify this in a model or at least in some accompanying markdown document.

swagger-specs/api.yaml の1707行目を確認し、PipelineExecutionStepState の details 定義を調査します。確認した environmentUrls と deploymentStepDescription のキーをモデルまたは付随する Markdown ドキュメントに記載し、API ユーザーにとって期待される構造が明確になるようにします。

索引モデルが issue の本文から書いたものです。