DOC dataset doc add return and improve doc #3315

Fixes #3315 
What do you think @arjoly ?
Yes, that would be right too. 
```
bunch : Bunch object with the attributes as follows
bunch.data : sparse matrix, shape [n_samples, n_features] - What data has description
bunch.target : array, shape [n_samples] - What target has description
bunch.target_names : list, length [n_classes] - What target_names has description 
```
Would this be a good format ? Because it would be good to have the description of each of the attributes as well.