cloud: update tidb_dml_type limitation (#22415)

[ti-chi-bot]

This is an automated cherry-pick of #22415

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[gemini-code-assist]

Summary of Changes

Hello @ti-chi-bot, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request updates the documentation to reflect limitations of the tidb_dml_type system variable within TiDB Cloud. Specifically, it clarifies that this variable is read-only for certain TiDB Cloud tiers, ensuring users are aware of these operational constraints.

Highlights

• Documentation Update: Added a note to the system-variables.md file clarifying that the tidb_dml_type variable is read-only for Starter and Essential tiers in TiDB Cloud.
• TiDB Cloud Feature Limitations: Updated the tidb-cloud/limited-sql-features.md file to include tidb_dml_type as a read-only system variable for TiDB Cloud.

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Changelog

• system-variables.md
  • Added a note specifying that tidb_dml_type is read-only for Starter and Essential TiDB Cloud tiers.
• tidb-cloud/limited-sql-features.md
  • Included tidb_dml_type in the table of limited SQL features, marking it as read-only.

Activity

• This pull request is an automated cherry-pick of an earlier change, cloud: update tidb_dml_type limitation #22415.

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[gemini-code-assist]

Code Review

This pull request updates the documentation for the tidb_dml_type system variable to note its read-only status on TiDB Cloud Serverless clusters. The changes correctly add this limitation to the system variable reference and the list of limited features on TiDB Cloud. I have provided one suggestion to improve the wording for better clarity and consistency.

[gemini-code-assist]

For better readability and consistency with other notes in this document, consider rephrasing this sentence.

Suggested change

	> This variable is read-only for [{{{ .starter }}}](https://docs.pingcap.com/tidbcloud/select-cluster-tier#starter) and [{{{ .essential }}}](https://docs.pingcap.com/tidbcloud/select-cluster-tier#essential).
	> For [{{{ .starter }}}](https://docs.pingcap.com/tidbcloud/select-cluster-tier#starter) and [{{{ .essential }}}](https://docs.pingcap.com/tidbcloud/select-cluster-tier#essential) clusters, this variable is read-only.

References

1. The documentation should be easy to understand for TiDB users. Rephrasing the sentence improves clarity and readability. ^(link)