What does this PR do?

This PR updates the logging documentation to match the current implementation. The old documentation referred to metrics like env/reward_mean, ppo/mean_scores, etc., but the actual metrics used in the current implementation are different, such as objective/scores, objective/rlhf_reward, etc.

I have updated the documentation to reflect the current metrics and added a reference to the PPO Trainer documentation for more detailed information.

Fixes #3437

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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  Pull Request section?
• Was this discussed/approved via a GitHub issue? Please add a link
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• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?

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