Update Redis documentation to document the precedence order
The following discussion from !162325 should be addressed: - [ ] @ashmckenzie started a [discussion](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/162325#note_2042147951): (+1 comment) > thought: As a follow-up so we can get this improvement merged, we should update https://docs.gitlab.com/omnibus/settings/redis.html to document the precedence order.
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