*: add two variables tidb_slow_log_rules and tidb_slow_log_max_per_sec

[zimulala]

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pingcap/tidb#64010
Add tidb_slow_log_rules and tidb_slow_log_max_per_sec user docs

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@zimulala: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-verify	9d5d049	link	true	/test pull-verify

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