Better save message for async checkpoint saving

[gobbleturk]

Description

Add a more accurate logging message for saving a checkpoint with an async checkpointer
Before: saved a checkpoint at step {step}
After: Started an asynchronous checkpoint save for step {step}

Tests

• Ran with an async checkpointer, saw expected message
• Ran with a sync checkpointer, saw expected message

Checklist

Before submitting this PR, please make sure (put X in square brackets):

• I have performed a self-review of my code.
• 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.