v8.5.7: update doc for max-user-connections (#20488)

[ti-chi-bot]

This is an automated cherry-pick of #20488

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• This PR is translated from: update doc for max_user_connections docs-cn#19898
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[gemini-code-assist]

Code Review

This pull request adds documentation for the newly supported MAX_USER_CONNECTIONS option in CREATE USER and ALTER USER statements, as well as the max_user_connections system variable introduced in TiDB v9.0.0. The review feedback points out unresolved git merge conflict markers in system-variables.md and a logical flow issue with the placement of code examples in sql-statement-create-user.md.

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[lilin90]

This PR is ready for merging.