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

See https://github.com/scikit-image/scikit-image/issues/1141
```
transformation object, callable ``outcoords = f(incoords, **kwargs)``, or ndarray
```
(Add "or" after last comma to make clear that these are all alternatives. Also, use a more descriptive name than "xy", since we want to phase out that coordinate notation except where we use a horizontal x and vertical y.)