Improve SFT documentation

[SurbhiJainUSC]

Description

Improve SFT documentation for single-host & multi-host TPUs.

Tests

N/A - documentation changes

Checklist

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

• 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.

[bvandermoon]

LGTM, thanks @SurbhiJainUSC. I think you mentioned offline that you already tested these updated instructions, right?

[SurbhiJainUSC]

LGTM, thanks @SurbhiJainUSC. I think you mentioned offline that you already tested these updated instructions, right?

Yes, I have tested this.