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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[Oreoxmt]

Suggested change

	> You will be billed for a Minimum Billing RCU, which is 25% (1/4) of your set Maximum RCU (or a minimum of 5,000 RCUs, whichever is higher). This floor ensures that dedicated resources are reserved to guarantee your peak performance. You will be charged this minimum amount even if your actual usage is lower.
	The maximum RCU value must be set in increments of 100.
	> - The maximum RCU value must be set in increments of 100.
	> - You will be billed for a minimum number of RCUs, known as the **Minimum Billing RCU**. This value is the greater of 25% of your configured maximum RCU or 5,000 RCUs. The minimum billing RCU ensures that dedicated resources are reserved for peak performance. You are charged this minimum amount even if your actual usage is lower.