[DOC] Add warning that *_Supervised versions interpret label -1 as "unlabeled"

[bellet]

In the following page of the doc :
http://contrib.scikit-learn.org/metric-learn/supervised.html#supervised-versions-of-weakly-supervised-algorithms

We should add a visible warning for users that supervised versions of weakly-supervised algorithms interpret the label -1 (which is commonly used in binary classification) as unlabeled.

[bellet]

Fixed in #266