sys-var: clarify tidb_slow_log_rules syntax/behavior and de-duplicate slow log docs (#22567)

[ti-chi-bot]

This is an automated cherry-pick of #22567

What is changed, added or deleted? (Required)

This PR refactors and clarifies tidb_slow_log_rules documentation in:

• system-variables.md
• identify-slow-queries.md

Main updates:

1. Reorganized tidb_slow_log_rules in system-variables.md with clearer structure:

   • Unified syntax and constraints
   • Type constraints
   • Full field table (type/unit/summary)
   • Effective behavior and matching order
   • Examples and usage recommendations

2. Clarified key behavior/constraints:

   • SESSION/GLOBAL rule capacity and separators
   • SESSION does not support Conn_ID (GLOBAL only)
   • Matching semantics (>= for numeric, = for string/bool)
   • Rule update behavior (SET overwrites, not appends)
   • Fallback to tidb_slow_log_threshold when no effective rules
   • String values cannot contain , / ; (quoted values still not supported)
   • Repeated fields in one rule: last value wins
   • Time-unit distinction (tidb_slow_log_rules time fields are seconds, threshold is milliseconds)

3. Reduced duplication between docs:

   • Kept rule contract/details in system-variables.md
   • Added metric interpretation section in identify-slow-queries.md
   • Replaced repeated long explanations with cross-references

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

• This PR is translated from: sys-var: clarify tidb_slow_log_rules syntax/behavior and de-duplicate slow log docs docs-cn#21337

• Fix issue: 完善 tidb_slow_log_rules 文档：字段/单位/取值范围（min-max）与语法约束 docs-cn#21324
• Other reference link(s): executor, variable: support cop_mvcc_read_amplification to trigger printing slow log tidb#66064

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

[ti-chi-bot]

@hfxsd This PR has conflicts, I have hold it.
Please resolve them or ask others to resolve them, then comment /unhold to remove the hold label.

[ti-chi-bot]

@ti-chi-bot: ## If you want to know how to resolve it, please read the guide in TiDB Dev Guide.

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[gemini-code-assist]

Code Review

This pull request introduces documentation for the new tidb_slow_log_rules and tidb_slow_log_max_per_sec system variables, which allow for multi-dimensional slow query log triggering and rate limiting. The review feedback identifies critical merge conflict markers left in the documentation files and provides numerous suggestions to align the text with the style guide, specifically regarding the use of the second person ("you"), active voice, and general grammatical clarity.

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