Update multiclass/multilabel documentation

Added examples to multiclass/label definitions

Reformulation of multilabel classification definition; updated list of multiclass algorithms

Added pointers for label binarizer helpers

Changed n_labels to n_classes
DOC overall improvements in the multiclass documentation
DOC take into account @vene and @ogrisel + specify format for multioutput-multiclass
DOC rewording
Typo
DOC ENH take into account @NelleV comments
DOC more comments from @NelleV