Document scope of 3D scatter depthshading.

Closes matplotlib#15657.

Author: timhoffm

lib/mpl_toolkits/mplot3d/axes3d.py

@@ -2239,7 +2239,8 @@ def scatter(self, xs, ys, zs=0, zdir='z', s=20, c=None, depthshade=True,
             For more details see the *c* argument of `~.axes.Axes.scatter`.
         depthshade : bool, optional, default: True
             Whether to shade the scatter markers to give the appearance of
-            depth.
+            depth. Each call to ``scatter()`` will perform its depthshading
+            independently.
         **kwargs
             All other arguments are passed on to `~.axes.Axes.scatter`.