[api] Undocumented option 'generated_from_issue' for the user story creation endpoint

[david-caro]

Moved from https://github.com/taigaio/taiga-back/issues/664

[brzuchal]

Are there any plans to add this info to the docs? The same is with external_reference it would be nice to know how may we use it using REST API, because it would be nice to have an external reference while integrating with a custom external bug tracker.