Check that backend-dependent kwarg's are properly documented

[ascillitoe]

In a number of places (such as GaussianRBF, and for detectors with backends), kwargs default to None, and then are set to backend-specific objects internally. We should survey all these instances to check that the logic is properly documented in the docstrings (see https://github.com/SeldonIO/alibi-detect/pull/497#discussion_r880406932).