v8.5.6 release notes: add info about cluster_id (#22766)

[ti-chi-bot]

This is an automated cherry-pick of #22766

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• This PR is translated from: v8.5.6 release notes: add info about cluster_id  docs-cn#21559
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[gemini-code-assist]

Code Review

This pull request updates the release notes for v8.5.6 by adding a section on system table changes, specifically documenting the addition of the cluster_id field to the mysql.tidb table. A review comment suggests improving the conciseness of the description by removing redundant phrasing regarding the read-only nature of the field and simplifying the sentence structure.

[gemini-code-assist]

The description is slightly wordy and contains redundant information. Since "read-only" already implies that the field cannot be modified, the phrase "and cannot be modified" is unnecessary. Additionally, "which represents" can be simplified to "which is" for better clarity and conciseness, following the repository's writing principles to avoid unnecessary words.

Suggested change

	| [`mysql.tidb`](https://docs.pingcap.com/tidb/v8.5/mysql-schema#cluster-status-system-tables) | Modified | Adds the `cluster_id` field, which represents the unique identifier of a TiDB cluster. Note that `cluster_id` is read-only and cannot be modified. |
	| [`mysql.tidb`](https://docs.pingcap.com/tidb/v8.5/mysql-schema#cluster-status-system-tables) | Modified | Adds the `cluster_id` field, which is the unique identifier of the TiDB cluster. This field is read-only. |

References

1. Avoid unnecessary words and repetition. ^(link)
2. Clarity, simplicity, completeness, and readability. ^(link)