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• replace the outdated SHOW GLOBAL VARIABLES LIKE 'tidb%' example with a shorter verified SHOW GLOBAL VARIABLES LIKE 'tidb_stmt_summary%' example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 simplifies the examples in the sql-statement-show-variables.md file by replacing the extensive list of all TiDB-specific variables with a shorter, focused example showing the TiDB statement summary global variables. The review feedback suggests a minor wording improvement to the introductory text of the example to ensure technical accuracy and better alignment with the documentation style guide.

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