premium: update minimum billing RCU

[qqqdan]

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[gemini-code-assist]

Code Review

This pull request updates the documentation for creating TiDB Premium instances, specifically clarifying the calculation and billing of Request Capacity Units (RCUs). The feedback suggests restoring bold formatting for UI elements and key terms like "Capacity" and "Minimum Billing RCU" to maintain consistency, as well as fixing a spacing issue between sentences.

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Timeline:

• 2026-04-24 08:54:26.305534531 +0000 UTC m=+2328871.510894578: ☑️ agreed by Oreoxmt.