Update Cascade documentation

[DN6]

What does this PR do?

There are some issues related to dtypes and torch versions with the Stable Cascade model. This PR:

1. Adds the appropriate information to the docs page to attempt to make it clear how the torch.bfloat16 dtype can be used with this model.
2. Fix some of the issues with the combined pipe docstrings.

Fixes # (issue)

Before submitting

• [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• [ ] Did you read the contributor guideline?
• [ ] Did you read our philosophy doc (important for complex PRs)?
• [ ] Was this discussed/approved via a GitHub issue or the forum? Please add a link to it if that's the case.
• [ ] Did you make sure to update the documentation with your changes? Here are the documentation guidelines, and here are tips on formatting docstrings.
• [ ] Did you write any new necessary tests?

Who can review?

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[HuggingFaceDocBuilderDev]

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