Document why N+1 Cached queries are considered bad

What does this MR do?

In &3873 (closed), we worked on fixing some end-points and providing the means to handle duplicated CACHE SQL calls.

We should document why duplicated Cached queries are considered bad form ~memory perspective, and update documentation https://docs.gitlab.com/ee/development/merge_request_performance_guidelines.html.

Related issues

#259003 (closed)

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