[BE][docs]Improve and update checkpoint documentation (#96862)
Updates:
- ~recommend user to use non-reentrant, mention that reentrant will be deprecated in the future~
- merges all the warnings into a single list of non-reentrant improvements over reentrant
- adds an additional entry to the list about allowing backward inside checkpointed region
Pull Request resolved: https://github.com/pytorch/pytorch/pull/96862
Approved by: https://github.com/albanD
