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

See https://github.com/scikit-image/scikit-image/issues/1141
I think it can be made understandable by saying something like: for each coordinate in the output image, one needs to specify the corresponding coordinate in the input.  E.g., coords[0, 10] tells us the (row, col) position of element [0, 10] in the input.

So, I guess I'm also suggesting that we use (rows, cols, 2) as the shape.