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

See https://github.com/scikit-image/scikit-image/issues/1141
I would put these comments *inside* the `if` clause. I think that might go against PEP8 but it is much more intuitive imho. (And you seem to agree with your new comments below...)

Sorry to bug you with another commit but I'm pretty sure it's my last comment!!! =)