Clarify DM foreign key support for one-to-one routing

[OliverS929]

What is changed, added or deleted? (Required)

• Clarify DM foreign key replication constraints when foreign_key_checks=1.
• Document that DM supports static one-to-one table routing for multi-worker foreign key causality, and still rejects route rules that map multiple source tables in the task to the same target table in this mode.
• Document that compact and multiple-rows are not supported when foreign_key_checks=1.
• Document hot configuration update restrictions when foreign_key_checks=1 and worker-count > 1.
• Add a schema correctness note recommending standard foreign key definitions before migration.

Which TiDB version(s) do your changes apply to? (Required)

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• v9.0 (TiDB 9.0 versions)
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• v6.5 (TiDB 6.5 versions)
• v6.1 (TiDB 6.1 versions)

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

• This PR is translated from: N/A
• Other reference link(s):
  • dm: make FK relation builder route-aware tiflow#12654
  • dm: support FK causality for one-to-one routes tiflow#12675
  • dm: reject FK causality hot config updates tiflow#12703
  • v8.5.6: dm experimentally supports foreign key #22616

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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Code Review

This pull request updates the DM documentation in dm-compatibility-catalog.md and dm-safe-mode.md to detail the behavior, requirements, and limitations of foreign key replication when foreign_key_checks=1 is enabled. The review feedback focuses on aligning the text with the repository style guide by converting passive voice sentences to active voice and ensuring consistent terminology, such as using "routing rules" instead of "route rules" and formatting configuration options with backticks.

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