Updated default of `tidb_stmt_summary_max_sql_length` from `4k` to `32k`

[mjonss]

What is changed, added or deleted? (Required)

Updated the doc, according to the change in https://github.com/pingcap/tidb/pull/63694

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

https://github.com/pingcap/tidb/pull/63694

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Do your changes match any of the following descriptions?

• [ ] Delete files
• [ ] Change aliases
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• [ ] Might cause conflicts after applied to another branch