gp_minimize callback arguments / return value

I think the callback argument for gp_minimize could be a bit more verbose about what these functions can accept as an in / output: https://github.com/scikit-optimize/scikit-optimize/blob/a2369ddbc332d16d8ff173b12404b03fea472492/skopt/optimizer/gp.py#L183

I think a link to / copy of the skopt/utils.py -> eval_callbacks documentation could help. 