Update Whisper docs clarifying inference support for long-form decoding

@ArthurZucker @patrickvonplaten 
I have updated the [Whisper docs page](https://huggingface.co/docs/transformers/v4.23.1/en/model_doc/whisper#transformers.WhisperProcessor) to clarify that the current `decode()` implementation doesn't support long-form yet. Hope it'll save folks some time, vs digging in to find that out :) 

_The documentation is not available anymore as the PR was closed or merged._<|||||>Thanks for the addition 😉 