sys-var: clarify tidb_slow_log_rules syntax/behavior and de-duplicate slow log docs (#21337)

[ti-chi-bot]

This is an automated cherry-pick of #21337

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)?

• 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 #21324
• Other reference link(s): executor, variable: support cop_mvcc_read_amplification to trigger printing slow log tidb#66064
• EN PR: sys-var: clarify tidb_slow_log_rules syntax/behavior and de-duplicate slow log docs docs#22567

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

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[ti-chi-bot]

@zimulala 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.

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