Update Docs: 17.0 Remove support for single database connection

What does this MR do and why?

From 17.0, we are soft-stopping instances from having a single main connection. We currently have a means for installations to override this requirement by removing the ci: section from config/database.yml, but we do not want installations to do this anymore, starting 17.0.

Therefore, it is important to get rid of mentions of this "override mechanism" from our documentation, which is what this MR does.

It also improves the warning message in installations where only 1 database connection is present.

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Related to #432391 (closed)