[Docs] Clarify how ingester replication happens

[callumj]

What this PR does

I naively was under the impression that replication functions like other systems where replication can happen when an ingester rejoins a memberlist which is incorrect. I figure it might be worth clarifying in the docs.

Which issue(s) this PR fixes or relates to

N/A

Checklist

• [ ] Tests updated
• [X] Documentation added
• [ ] CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

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