[PostGIS] #5867: Add documentation for Straight Skeleton with Point Touching Rings

[PostGIS]

#5867: Add documentation for Straight Skeleton with Point Touching Rings
--------------------------+---------------------------
 Reporter:  gravitystorm  |      Owner:  pramsey
     Type:  enhancement   |     Status:  new
 Priority:  medium        |  Milestone:  PostGIS 3.5.3
Component:  postgis       |    Version:  3.4.x
 Keywords:                |
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 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.
-- 
Ticket URL: <https://trac.osgeo.org/postgis/ticket/5867>
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