[WHISPER] Update documentation of processor and nits in forward example
# What does this PR do?

Fixes #19672

With respect to #19672, the `__call__` method of the `WhisperProcessor` was updated. 
The doctest were also not passing with the new `max_length`, see #19670 and #19668 
@ydshieh 
_The documentation is not available anymore as the PR was closed or merged._