Fix docstrings to be less confusing about pixel coordinates.

As discussed in #1140, the old nomenclature was confusing.  
I ran  `grep -irn "(x,y)" .` and `grep -irn "(x, y)" .`, and these were the only two places where a docstring referred to (x,y) coordinates in an image.  

`skimage/morphology/_pnpoly.pyx:59` `points_inside_poly` uses (x, y) coordinates, but is not specifically linked to an image.
The same is true of `skimage/measure/fit.py` which uses (x, y) in several places.