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Complete redis Sentinel documentation in regards to authentication

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# Checklist * [x] I have checked the [issues list](https://github.com/celery/celery/issues?utf8=%E2%9C%93&q=is%3Aissue+label%3A%22Category%3A+Documentation%22+) for similar or identical bug reports. * [x] I have checked the [pull requests list](https://github.com/celery/celery/pulls?q=is%3Apr+label%3A%22Category%3A+Documentation%22) for existing proposed fixes. *...