Update OutlierVAE `score_type` docs

[jklaise]

The main docs say that only mse is supported, but the API docs say that both mse and proba are supported.

Edit: I see in the source code it is not actually supported yet, but we should update the API docstring to reflect this otherwise it's misleading, also probably worth dropping proba from a valid option for now as the current logic seems to be broken: https://github.com/SeldonIO/alibi-detect/blob/04c0574d98c5ad0ac9e3c4376ce4e91c456bfbde/alibi_detect/od/vae.py#L190-L191