PyDoc: Document the undo argument when calling operators

Ref !160359

doc/python_api/examples/bpy.ops.4.py

@@ -0,0 +1,24 @@
+"""
+.. _operator-undo:
+
+Undo
+----
+
+The optional second positional argument is a boolean that controls whether the
+operator call is pushed onto the undo stack.
+
+It defaults to ``False``, so operators run from a script do not add to the undo
+history. Pass ``True`` when you want the call to be undoable, for instance when
+a script performs an edit the user should be able to undo interactively.
+
+If this call happens while nested inside another operator call, the argument
+has no effect: the outer call already suppresses undo for everything nested
+inside it, and that only lifts once the outer call returns.
+
+Like the execution context this argument is optional, but the positional order
+must be kept, so the execution context is given first.
+"""
+
+# Add a cube as an undoable operator call.
+import bpy
+bpy.ops.mesh.primitive_cube_add('EXEC_DEFAULT', True)