cloud: clarify serverless private endpoint VPC scope

[qiancai]

What is changed, added or deleted? (Required)

Clarify the customer-side scope of private endpoints for TiDB Cloud Starter or Essential instances over AWS PrivateLink:

• Clarify that for each VPC in an AWS Region, one private endpoint is enough for all Starter or Essential instances in that Region from the same VPC.
• Clarify that private endpoints cannot be shared across VPCs.
• Clarify the duplicate private DNS error tip by mentioning the existing endpoint is in the same VPC.

Which TiDB version(s) do your changes apply to? (Required)

• master (the latest development version)
• v8.5 (TiDB 8.5 versions)
• v8.4 (TiDB 8.4 versions)
• v8.3 (TiDB 8.3 versions)
• v8.2 (TiDB 8.2 versions)
• v8.1 (TiDB 8.1 versions)
• v7.5 (TiDB 7.5 versions)
• v7.1 (TiDB 7.1 versions)
• v6.5 (TiDB 6.5 versions)
• v6.1 (TiDB 6.1 versions)
• v5.4 (TiDB 5.4 versions)
• v5.3 (TiDB 5.3 versions)

What is the related PR or file link(s)?

• This PR is translated from:
• Other reference link(s):
  • https://docs.pingcap.com/zh/tidbcloud/set-up-private-endpoint-connections-serverless/?plan=starter
  • cloud: update serverless tier private link instructions #13139

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

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[gemini-code-assist]

Code Review

This pull request updates the documentation for setting up private endpoint connections for TiDB Cloud Serverless, specifically clarifying that private endpoints are scoped to a VPC and cannot be shared across VPCs. The review feedback focuses on improving clarity and readability by recommending the use of active voice and the second person, as well as removing redundant phrasing to better align with the style guide.

[gemini-code-assist]

To improve clarity and readability, avoid passive voice and redundant phrasing. The phrase 'which can be used by' is passive, and 'in the same Region' is repeated unnecessarily in the same sentence.

Suggested change

> For each VPC in an AWS Region, you only need to create one private endpoint. The endpoint can be used by all {{{ .starter }}} or {{{ .essential }}} instances in that Region from the same VPC, but cannot be shared across VPCs.

> {{{ .starter }}} or {{{ .essential }}} instances in the same AWS Region use the same TiDB Cloud PrivateLink endpoint service. In your AWS account, each private endpoint is scoped to a specific VPC. For each VPC in that Region, you only need to create one private endpoint. All {{{ .starter }}} or {{{ .essential }}} instances in that Region can then use this endpoint from the same VPC. You cannot share a private endpoint across VPCs.

References

1. Avoid passive voice overuse. ^(link)
2. Avoid unnecessary words and repetition. ^(link)

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