[FSDP] Clarify `MixedPrecision` docs (#91974)
New docs:
![Screen Shot 2023-01-10 at 8 07 19 PM](https://user-images.githubusercontent.com/31054793/211694428-c8ebf210-85c5-4b8a-a174-ee8022d8b8fd.png)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/91974
Approved by: https://github.com/zhaojuanmao