docs: use a shorter SHOW VARIABLES example

[doc-claw-bot]

What is changed, added or deleted? (Required)

• replace the outdated SHOW GLOBAL VARIABLES LIKE 'tidb%' example with a shorter verified SHOW GLOBAL VARIABLES LIKE 'tidb_stmt_summary%' example
• avoid the disputed tidb_wait_split_region_finish row in the example output
• keep the other SQL examples unchanged

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

• master (the latest development version)
• v9.0 (TiDB 9.0 versions)
• v8.5 (TiDB 8.5 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)

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

• This PR is translated from:
• Other reference link(s):
  • Verified against a local TiDB v8.5.6 instance for the replacement example query.

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 sql-statement-show-variables.md documentation by truncating the long example output of SHOW GLOBAL VARIABLES LIKE 'tidb%' to a shorter, representative list, and updating the introductory text. The reviewer suggested a minor phrasing improvement to change "part of the TiDB-specific global variables" to "some of the TiDB-specific global variables" to enhance readability and use more idiomatic technical English.

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[gemini-code-assist]

To improve readability and use more idiomatic technical English, consider changing "part of the TiDB-specific global variables" to "some of the TiDB-specific global variables".

Suggested change

	The following example shows part of the TiDB-specific global variables. For detailed descriptions, refer to [System Variables](/system-variables.md).
	The following example shows some of the TiDB-specific global variables. For detailed descriptions, refer to [System Variables](/system-variables.md).

References

1. Clarity, simplicity, completeness, and readability are key aspects of documentation review. ^(link)