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

See https://github.com/scikit-image/scikit-image/issues/1141
Which is a callable, as I see below... Yes, please add a comment specifying this next to "else". It may even be worth adding one more "if" to make sure that this is indeed the case, or there's going to be a very confusing error message downstream if someone passes in a different type.