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

See https://github.com/scikit-image/scikit-image/issues/1141
Parsing this requires some mental overhead.  Why not split it as follows:

```
For 2-D images:

- Method one
- Method two

For N-D images (including 2-D images):

- Method three
- Method four
```