Update missing docs on `activation_dropout` and fix DropOut docs for SEW-D
Author: gau-nernst
Pull request: yes

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Fixes #25854

Add doc for `activation_dropout` for various audio models. Let me know if I miss out any.

For SEW-D, document the behavior that `hidden_dropout` is not used, while `activation_dropout` acts more like `hidden_dropout` in other models.

On a side note, it will be good if there is a test to catch undocumented config attributes in the future.

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Comments:
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