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

See https://github.com/scikit-image/scikit-image/issues/1141
Just wrote the truth table for `isinstance()` and `.shape ==`, and I get the same outcomes for all boxes (assuming you add an `else:` at the end of my `if isinstance` clause to cover the non-array case, below). Where do you think we differ? (ie in which case will my control flow diverge from yours?)