[functorch] Add more details to the functorch install page (#86823) (#86903)
Added some details about:
- `pip uninstall functorch` being helpful if there are problems
- `pip install functorch` still working for BC reasons.
Test Plan:
- wait for docs preview
Pull Request resolved: https://github.com/pytorch/pytorch/pull/86823
Approved by: https://github.com/samdow
