Improving plt.hist documentation

Adds a description of the return value of `plot.hist()`. Suggested by tcaswell in [this SO post](http://stackoverflow.com/questions/23773131/what-are-n-bins-and-patches-in-matplotlib/23778929?noredirect=1#comment36584893_23778929). I have tried my best to stick to the numpy doc format but admit I was confused as to if this should go in the `return` statement or the extended description at the top of the docstring.