COSMIT smipler & more robust check in test_kfold_valueerrors

Using assert_warn_message internally cleans the warning
registries making this test more robust to changes in other tests.
DOC fix see also references
Merge pull request #3491 from ogrisel/fix-warning-test-cross-val

[MRG] TST unstable warning test in test_kfold_valueerrors
DOC fix see also reference
DOC fix markup error
MAINT fix prng in test_f_oneway_ints

Not fixing the PRNG cause the test to fail randomly under windows
where the 4 decimals place check can be too strict for some unlucky
random draws of test data.
ENH Stochastic Coordinate Descent for ElasticNet & Lasso
FIX Implemented correct handling of multilabel y in cross_val_score
FIX Support unseen labels LabelBinarizer and test

This fix restores the default behavior of 0.14.1 for backward
compat.
Add sample_weight parameter to metrics.jaccard_similarity_score

 - Include this metric in sample_weight test in tests/test_common.py
DOC whats_new.rst: missing backported fixes for 0.15.1