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

See https://github.com/scikit-image/scikit-image/issues/1141
I don't like seeing this repeated, but others may differ in opinion. I would write:

```python
if isinstance(inverse_map, np.ndarray):
    if inverse_map.shape == (3, 3):
        inverse_map = ProjectiveTransform(matrix=inverse_map)
    else:
        coords = inverse_map
```