DOC: stats: Add 'entropy' to the stats package-level documentation.

It was noted (indirectly) in an answer to http://stackoverflow.com/questions/22097409/kl-divergence-of-continuous-pdfs that `entropy` is not in the scipy.stats module docstring.

I put it at the end of the list, because its arguments are qualitatively different from most of the other statistical functions in the module.