Reword "Reordering is not commutative" phrase in tutorial.

- "Commutative" is a bit jargony, and technically applies to *binary*
  operations (see e.g. wikipedia), not to "reordering".
- The fact that drawing (with or without alpha) is order-dependent is
  not something specific to Matplotlib.

Author: anntzer

tutorials/colors/colors.py

@@ -103,7 +103,7 @@
 
 .. note::
 
-    Re-ordering Artists is not commutative in Matplotlib.
+    Changing the order of Artists will generally change the resulting figure.
 
 
 "CN" color selection