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.

Review swagger-specs/api.yaml at line 1707 and inspect the PipelineExecutionStepState details definition. Document the observed environmentUrls and deploymentStepDescription keys in the model or an accompanying Markdown document, with the expected structure clear to API users.

Written by the indexing model from the issue text.