Add documentation for Straight Skeleton with Point Touching Rings

[gravitystorm]

I have recently found that CG_StraightSkeleton (and CG_ApproximateMedialAxis) will throw an error for certain valid geometries, specifically valid polygons with a point touching inner ring.

gis=# select st_isvalid(ST_GeomFromText('POLYGON((0 0, 3 0, 3 3, 0 3, 0 0),(0 0, 1 2, 2 1, 0 0))'));
 st_isvalid 
------------
 t
(1 row)

gis=# select st_straightskeleton(ST_GeomFromText('POLYGON((0 0, 3 0, 3 3, 0 3, 0 0), (0 0, 1 2, 2 1, 0 0))'));
NOTICE:  During straight_skeleton(A) :
NOTICE:    with A: POLYGON((0/1 0/1,3/1 0/1,3/1 3/1,0/1 3/1,0/1 0/1),(0/1 0/1,1/1 2/1,2/1 1/1,0/1 0/1))
ERROR:  straight skeleton of Polygon with point touching rings is not implemented.

I think it would be helpful to mention this in the documentation for these functions.

This is in order to raise awareness of the limitation when a developer (like me!) is implementing the query, rather than it failing unexpectedly some time later with production data.