Issue #16863: Explain difference between text and 2.7 behavior.

Doc/howto/argparse.rst

@@ -8,6 +8,8 @@ Argparse Tutorial
 
 This tutorial is intended to be a gentle introduction to :mod:`argparse`, the
 recommended command-line parsing module in the Python standard library.
+This was written for argparse in Python 3. A few details are different in 2.x,
+especially some exception messages, which were improved in 3.x.
 
 .. note::