DOC dataset doc add return and improve doc #3315

Fixes #3315 
What do you think @arjoly ?
I see. One other alternative to avoid this problem would be write them like this but all the attributes will be in the description of the main bunch object.

Returns 
-----------
bunch : Bunch object
        bunch.data: sparse matrix, shape [n_samples, n_features]
        bunch.target: array, shape [n_samples]
        bunch.target_names: list, length [n_classes]

And also can include the description of each attribute on the same after.