improve doc around supported tasks and accelertor options

[rbrugaro]

fix documentation

Prior #739 removes 'ipex' as default accelerator and
seems is the only supported one in MAPPING_LOADING_FUNC. removed inc and openvino from options

also, updated list of supported pipeline tasks to match those of optimum/intel/ipex/modeling_base.py

Fixes # 742

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• [x ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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[rbrugaro]

@echarlaix

[echarlaix]

Thanks for fixing the docstrings @rbrugaro