Document tuning of lower Geo concurrency default settings

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What does this MR do and why?

This MR adds a remark to the Geo tuning guide to inform users about the lowered concurrency default settings. As suggested in #24081 (closed), the actual DB migration changes to introduce the lower defaults are handled in a separate MR (see !184473 (merged)).

🔧 with at Adfinis

References

• #24081 (closed)
• !184473 (merged)
• (#393570)

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