added warning to matrix input for kdeplot.
added warning to matrix input for kdeplot.

phobson/seaborn,lukauskas/seaborn,mwaskom/seaborn,lukauskas/seaborn,anntzer/seaborn,mwaskom/seaborn,phobson/seaborn,arokem/seaborn,anntzer/seaborn,arokem/seaborn,sauliusl/seaborn,petebachant/seaborn