DOC explain SelectorMixin strategy with multiple classes.
DOC Add note about sample weights in export_graphviz.
Merge pull request #3829 from amueller/feature_selection_from_model_doc

DOC explain SelectorMixin strategy with multiple classes.
unused variables are redefined later
FeatureUnion example using a heterogeneous datasource.

Uses the 20newsgroups dataset. Extracts & processes the subject-line and body
with separate pipelines and combines them with a FeatureUnion.
DOC RidgeCV more explicit documentation of CV parameter, taken from GridSearchCV.