[functorch][docs] Downgrade the warning about forward-mode AD coverage (#87383)
Previously we claimed that "forward-mode AD coverage is not that good".
We've since improved it so I clarified the statement in our docs and
downgraded the warning to a note.
Test Plan:
- view docs
Pull Request resolved: https://github.com/pytorch/pytorch/pull/87383
Approved by: https://github.com/samdow
