Issue #16863: Explain difference between text and 2.7 behavior.
diff --git a/Doc/howto/argparse.rst b/Doc/howto/argparse.rst
index f110a55..f113c98 100644
--- a/Doc/howto/argparse.rst
+++ b/Doc/howto/argparse.rst
@@ -8,6 +8,8 @@
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::