Update docs to point to new sdpa_kernel context manager (#121180)
# Summary
Updates the SDPA docs to fix some small inaccuracies and points to the new sdpa_kernel context manger. The Enum like type binded from cpp SDPBackend does not render its fields for some reason. Manually list them instead for now
Pull Request resolved: https://github.com/pytorch/pytorch/pull/121180
Approved by: https://github.com/mikaylagawarecki
