ENH + DOC set a default scorer in the multiclass module

follow the discussion at the sprint
and intend to fix https://github.com/scikit-learn/scikit-learn/issues/1979


I think that here you have to have a "shape = (n_samples, ) or shape ) (n_samples, n_labels)" and to explain that n_labels is defined only in the multilabel case (I am thinking that people who don't have multilabel problems have no clue what this means, and will be a bit surprised.