DOC: boxplot docs do not explain what the plot shows at all
Extensive documentation is probably not needed, but at a minimum, the docstring should say
- what the red line is (median)
- what the box shows (quartiles)
- what the whiskers show (?)
- by what rules the outliers are shown or not

A link to wikipedia and/or matplotlib docs would be nice too.

I could try to contribute this, if it is deemed useful.
