What does this PR do?

The following documentation improvements are made in this PR:

1. Added docstring notes to num_sparse_encoder_layers and num_sparse_decoder_layers parameter SwitchTransformersConfig explaining that when set to 0 with a single layer model, the current implementation may still create a sparse layer due to the sparse step calculation. This edge case is not encountered in existing checkpoints.
2. Updated an outdated comment in audio_utils.py that stated spectrogram() does not support batching, even though spectrogram_batch() already exists. Changed to a note referencing the batch function.

Fixes #43335.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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  Pull Request section?
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  [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and
  [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
• Did you write any new necessary tests?

cc: @Rocketknight1