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

See https://github.com/scikit-image/scikit-image/issues/1141
I'm not sure because I've never used these functions, and have no idea how someone would generate the `coords` ndarray... Others can comment. At the very least, you might take a performance hit. At worst, depending on the tools being used upstream of this, you'll need to transpose first only to have the function transpose again inside.