bpo-39392: Turtle overlap fill depends on OS (GH-18223)

Whether or not overlap regions for self-intersecting polygons
or multiple shapes are filled depends on the operating system graphics,
typeof overlap, and number of overlaps.
(cherry picked from commit 2824c45)

Co-authored-by: Terry Jan Reedy <[email protected]>

Author: miss-islington

Doc/library/turtle.rst

@@ -1013,6 +1013,11 @@ Filling
 
    Fill the shape drawn after the last call to :func:`begin_fill`.
 
+   Whether or not overlap regions for self-intersecting polygons
+   or multiple shapes are filled depends on the operating system graphics,
+   type of overlap, and number of overlaps.  For example, the Turtle star
+   above may be either all yellow or have some white regions.
+
    .. doctest::
 
       >>> turtle.color("black", "red")

Misc/NEWS.d/next/Documentation/2020-01-27-18-18-42.bpo-39392.oiqcLO.rst

@@ -0,0 +1 @@
+Explain that when filling with turtle, overlap regions may be left unfilled.