expose useful _plot_img_with_bg kwargs explicitly in plotting.plot_img

[jeromedockes]

plotting.plot_img helpfully forwards kwargs, and in particular bg_img, to
plotting.img_plotting._plot_img_with_bg.

However the docstring says kwargs are passed to pyplot.imshow.

I would suggest exposing important arguments such as vmin, vmax and bg_img
explicitly in the function signature.

note that this example:

http://nilearn.github.io/auto_examples/02_decoding/plot_haxby_searchlight.html#sphx-glr-auto-examples-02-decoding-plot-haxby-searchlight-py

uses the bg_img kwarg

[bthirion]

+1

[jeromedockes]

+1

then I'll open a PR