DOC: ylabels for secondary_y axis
modified "Selective Plotting on Secondary Y-axis" example to show how to set ylabels for both left and right y axis

good, but I don't think it's playing nice with the sphinx machinery
because regenerating the docs doesn't produce the labels. Have a look?
 Should be fixed now. The plot below had the same filename. 
 much appreciated. merged as dde093e.
