Update PointSize doc

Note the values of defaults.

mathics/builtin/graphics.py

@@ -998,12 +998,16 @@ class Thick(Builtin):
 class PointSize(_Size):
     """
     <dl>
-    <dt>'PointSize[$t$]'
-        <dd>sets the diameter of points to $t$, which is relative to the overall width.
+      <dt>'PointSize[$t$]'
+      <dd>sets the diameter of points to $t$, which is relative to the overall width.
     </dl>
+    'PointSize' can be used for both two- and three-dimensional graphics. The initial default pointsize is 0.008 for two-dimensional graphics and 0.01 for three-dimensional graphics.
 
     >> Table[Graphics[{PointSize[r], Point[{0, 0}]}], {r, {0.02, 0.05, 0.1, 0.3}}]
      = {-Graphics-, -Graphics-, -Graphics-, -Graphics-}
+
+    >> Table[Graphics3D[{PointSize[r], Point[{0, 0, 0}]}], {r, {0.05, 0.1, 0.8}}]
+    = {-Graphics3D-, -Graphics3D-, -Graphics3D-}
     """
 
     def get_size(self):