add missing doc

[felipemello1]

Context

What is the purpose of this PR? Is it to

• [ ] add a new feature
• [ ] fix a bug
• [x ] update tests and/or documentation
• [ ] other (please add here)

https://github.com/pytorch/torchtune/issues/1846

made it to look the same as in preference dataset: https://github.com/pytorch/torchtune/blob/09c2619f713e771b4159f7b83bac8971c7053bd3/torchtune/datasets/_preference.py

[pytorch-bot[bot]]

:link: Helpful Links

:test_tube: See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/torchtune/1924

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As of commit 6c0a05b39555ae53203c2c31ff60055fa1ae6c5d with merge base 09c2619f713e771b4159f7b83bac8971c7053bd3 (): :green_heart: Looks good so far! There are no failures yet. :green_heart:

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