Request to update guide on how to export/download Custom QA knowledge bases via REST API.

> Please provide us with the following information:
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### This issue is for a: (mark with an `x`)
```
- [ ] bug report -> please search issues before submitting
- [ ] feature request
- [X] documentation issue or request
- [ ] regression (a behavior that used to work and stopped in a new release)
```

### Minimal steps to reproduce
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Please navigate to the following Custom QA REST API links.

When exporting Custom QA KB, the following APIs are issued.

1) Custom QA Export API to export knowledge base
https://learn.microsoft.com/en-us/rest/api/cognitiveservices/questionanswering/question-answering-projects/get-export-status?tabs=HTTP

2) The above API returns a URL in the operation-location header. Then, this may be used as a parameter to issue the Custom QA Get Export Status API to get the resultURL for download.
https://learn.microsoft.com/en-us/rest/api/cognitiveservices/questionanswering/question-answering-projects/get-export-status?tabs=HTTP

In the 2nd step, it mentions that the purpose of Get Export Status API is to _"get the status of an Export job, once job completes, returns the project metadata, and assets.", which may be confusing for the user as the 2nd step only returns a resultURL for download. Then, an additional API request needs to be issued by using the resultURL value and subscription key. The additional step cannot be used to download the file directly from a browse for example, but it requires a tool like Postman to **Send and Download** the result to file. Or, by clicking **Send** to send the request and **Save Response** when a valid response is received. Please see the sample image below.


Thank you and hope you may consider the above suggestions for future enhancements!

### Any log messages given by the failure
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### Expected/desired behavior
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### OS and Version?
> Windows 7, 8 or 10. Linux (which distribution). macOS (Yosemite? El Capitan? Sierra?)

### Versions
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### Mention any other details that might be useful

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> Thanks! We'll be in touch soon.