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

See https://github.com/scikit-image/scikit-image/issues/1141
@stefanv, this is because of a prior discussion on this PR, and related discussions elsewhere. If we agree to maintain a consistent notation throughout the package, then we would use (row, col) or rc or rrcc for:
```
        col
    +------------->
    |
    |
    |
    |
row V
```

and XY for:

```
Y   ^
    |
    |
    |
    |
    +------------->
        X
```

But in this case, the input is in col, row order, meaning origin at the top-left corner of the array. Hence, CR.