Update what's new
FIX #3503: use linalg.pinv to better deal with singular input data
Fix using ground truth in silhouette score in document clustering. Closes #3806.
DOC explain SelectorMixin strategy with multiple classes.
Sample weight support for hinge loss
COSMIT: Use searchsorted in weighted_percentile
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