Clarify that monorepos may keep reads from getting distributed

What does this MR do and why?

Related ticket: https://gitlab.zendesk.com/agent/tickets/401487

In some large customer, large monorepo environments with heavy CI usage we've seen distributed reads enabled, but all traffic being served by primary. This is expected, so the docs call out that example to hint to that case and make it easier for customers + support engineers to quickly point to.

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