Clarify RL checkpoint environment variables

[huytransformer]

Description

Clarifies the checkpoint setup in the single-host RL tutorial. The tutorial now distinguishes the MaxText model name from the converted MaxText checkpoint path, and notes that the checkpoint step sets MAXTEXT_CKPT_PATH for load_parameters_path.

Checklist

• I have performed a self-review of my code. For an optional AI review, add the gemini-review label.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have run end-to-end tests tests and provided workload links above if applicable.
• I have made or will make corresponding changes to the doc if needed, including adding new documentation pages to the relevant Table of Contents (toctree directive) as explained in our documentation.