Improve desciption of inverse_map and add option to direclty pass coordi...

See https://github.com/scikit-image/scikit-image/issues/1141
I corrected this description. It turns out that the current implementation does not allow to define a callable for N-D input, but only for 2-D.  The problem is that the current implementation automatically warps images with color images, but only passes a (N, 2) array to the callable.

I would keep it as is. Otherwise, imo we would have to add a new parameter, that specifies whether a 3-D image should be interpreted as color or real 3-D...