[FSDP][BE][Docs] Improve auto wrap policy doc (#78400)
Closes https://github.com/pytorch/pytorch/issues/78399
- Add expected type of callable
- Clarify what the policy function should return, and how its used (i.e. what's done depends on `recurse` flag).
Pull Request resolved: https://github.com/pytorch/pytorch/pull/78400
Approved by: https://github.com/awgu
