Clarify `use_fast` parameter in docstring
This PR addresses the ambiguity of the `use_fast` parameter raised in #20817.
_The documentation is not available anymore as the PR was closed or merged._<|||||>> what happens if the architecture supports it but the model doesn't?

Hmm that's a good question! Do you happen to know if any of the other architectures have this issue or if it is just a bug with OPT? I'll remove the suggestion to check the supported framework list so we don't end up confusing anyone.<|||||>It's the first time I run into this inconsistency across models of the same arch, but I have never needed to ensure it was `fast` before so who knows it may have happened a lot and I wasn't the wiser.

So +1 to remove the suggestion unless we somehow can stand behind it.<|||||>This was the bug for OPT, so it shouldn't happen for other models.