warning log in RLTrainer

[willccbb]

Description

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Test improvement

Testing

• All existing tests pass when running uv run pytest locally.
• New tests have been added to cover the changes

Checklist

• My code follows the style guidelines of this project as outlined in AGENTS.md
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published

Additional Notes

---

Note

Clarifies usage of vf.RLTrainer and nudges toward prime-rl.

• Adds a logger.warning in RLTrainer.__init__ stating it is for educational/demo purposes only and linking to prime-rl
• Updates docs/training.md to include the same note under the vf.RLTrainer section
• Adds a clarifying inline comment in MultiTurnEnv.rollout about @vf.stop/@vf.cleanup behavior

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