Enhance max_execution_time documentation with link

[MikeShi42]

Updated the explanation of the max_execution_time setting to include a link to the documentation. Also clarify it's in seconds.

Summary

Checklist

• Delete items not relevant to your PR
• URL changes should add a redirect to the old URL via https://github.com/ClickHouse/clickhouse-docs/blob/main/docusaurus.config.js
• If adding a new integration page, also add an entry to the integrations list here: https://github.com/ClickHouse/clickhouse-docs/blob/main/docs/integrations/index.mdx

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