docs: clarify reasoning-only template guidance

[he-yufeng]

Summary

• clarify the reasoning-template warning so _nothink is recommended only when a non-reasoning template exists
• document the reasoning-only path (template: qwen3_6 + enable_thinking: false) in both English and Chinese dataset READMEs
• keep template behavior unchanged

Fixes #10566.

Tests

• python -m py_compile src/llamafactory/data/template.py
• python -m ruff check src/llamafactory/data/template.py
• Select-String -Path data\README.md,data\README_zh.md,src\llamafactory\data\template.py -Pattern "qwen3_6|enable_thinking: false|qwen3_vl_nothink" -Encoding UTF8
• git diff --check

[gemini-code-assist]

Code Review

This pull request updates the documentation in both English and Chinese README files, as well as the warning message in template.py, to clarify the usage of reasoning templates. Specifically, it explains that the _nothink template suffix should only be used when a separate non-reasoning template is available, whereas for reasoning-only model families, users should keep the reasoning template and set enable_thinking: false to match non-thinking inference. There are no review comments, so I have no feedback to provide.

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