bpo-39392: Turtle overlap fill depends on OS (#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.
diff --git a/Doc/library/turtle.rst b/Doc/library/turtle.rst
index 7f9f0c3..fed8504 100644
--- a/Doc/library/turtle.rst
+++ b/Doc/library/turtle.rst
@@ -1051,6 +1051,11 @@
 
    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::
       :skipif: _tkinter is None