[ckpt] fix: edit esi doc

[plutoZZZZ]

What does this PR do?

This PR addresses the "ESI" comprehension issue left by the previous PR (#2192).
This PR refines ppo_trainer.yaml by expanding the esi_redundant_time comment to define ESI (Elastic Server Instance) and draw a parallel to a training plan. In ray_trainer.py, it clarifies ESI-related checkpoint-saving conditions. These edits boost code readability and maintainability.

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[gemini-code-assist]

Code Review

This pull request focuses on improving code clarity and maintainability by adding descriptive comments to the ppo_trainer.yaml configuration file and the ray_trainer.py script. The changes specifically clarify the meaning of 'ESI' (Elastic Server Instance) and the conditions for saving checkpoints related to ESI expiration. Based on the specified review criteria and severity levels (high, critical), no issues requiring immediate attention or posing significant problems were identified in these documentation-focused changes.