BUG: Make secondary_y work properly for bar plots GH3598
As far as I'm aware this seems to work properly, but I'm not sure if this was done the previous way for a reason I don't understand.

Fixes: https://github.com/pydata/pandas/issues/3598

can u add release notes?
 Done
 @jreback looks ok 2 me, thoughts?
 is mark_right a _public_ option?
 In a theoretical sense they should be public, in the spirit of this option being similar to "stacked" for bar plots or the same properly "mark_right" for a LinePlot; it is a perfectly reasonable publicly editable properly of a plot.

However, since via standard use of (series|frame).plot(kind='xyz'), you never actually get to interact with the XyzPlot object (and nowhere else in the pandas codebase are these plot objects ever interacted with) I don't think it matters all that much, and since there already is a previous convention I believe sticking with that is reasonable.
 that's all fine
didn't know if they belong in the doc string
 I just added it to the docstring of plot_frame() [along with fixing minor mistake in secondary_y].  It is getting a little verbose, but more documentation is probably better than less in general.  Up to you.
 that looks good thxs
 thanks!
