premium: update minimum billing RCU (#22730)

Author: qqqdan

tidb-cloud/premium/create-tidb-instance-premium.md

@@ -43,13 +43,12 @@ If you have the `Organization Owner` role, you can create a {{{ .premium }}} ins
 2. On the **Create Resource** page, select **Premium** as your plan.
 3. Enter a name for your {{{ .premium }}} instance, and then choose a cloud provider and a region where you want to host your instance.
 4. (Optional) To group this {{{ .premium }}} instance in a project for management, click **Group Your Instance in a Project**, and then select the target project for the instance. If there is no project in your organization, you can create one by clicking **Create a Project**.
-5. In the **Capacity** area, set the maximum number of the Request Capacity Units (RCUs) for your instance.
-
-    RCUs represent the compute resources provisioned for your workload. TiDB Cloud automatically scales your instance within this range based on demand.
+5. In the **Capacity** area, set the maximum number of Request Capacity Units (RCUs) for your instance. RCUs represent the compute resources provisioned for your workload. TiDB Cloud automatically scales your instance within this range based on demand.
 
     > **Note:**
-    >
-    > You will be billed for the **Minimum Billing RCU** shown below the maximum number of RCUs, 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 and 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.
 
 6. For {{{ .premium }}} instances, only regional high availability is enabled, and it is not configurable. For more information, see [High Availability](/tidb-cloud/serverless-high-availability.md).