Python API Docs: explain the CANCELLED return code of operators.

The effect of CANCELLED on the undo stack is quite obscure, and
mistakenly using it after doing some changes causes confusing
behavior. It's better to describe it explicitly in the docs.

doc/python_api/examples/bpy.ops.py

@@ -10,8 +10,9 @@ Only keyword arguments can be used to pass operator properties.
 Operators don't have return values as you might expect,
 instead they return a set() which is made up of:
 ``{'RUNNING_MODAL', 'CANCELLED', 'FINISHED', 'PASS_THROUGH'}``.
-Common return values are ``{'FINISHED'}`` and ``{'CANCELLED'}``.
-
+Common return values are ``{'FINISHED'}`` and ``{'CANCELLED'}``, the latter
+meaning that the operator execution was aborted without making any changes or
+saving an undo history entry.
 
 Calling an operator in the wrong context will raise a ``RuntimeError``,
 there is a poll() method to avoid this problem.