Document enable_autocheckpoint and its distinction from emergency checkpointing.

[abhinavclemson]

Description

Document enable_autocheckpoint and its distinction from emergency checkpointing.

Adds enable_autocheckpoint to the checkpointing guides, explaining that it enables saving a checkpoint upon receiving a SIGTERM signal. Clarifies the difference between this reactive autocheckpointing and the proactive emergency checkpointing.

Checklist

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• 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.

[richjames0]

lgtm