[3.14] pythongh-76637: Note that undefined Codec is for testing (pythonGH-136531) (python#136536)

pythongh-76637: Note that `undefined` Codec is for testing (pythonGH-136531)

Closes pythonGH-76637
(cherry picked from commit 975b57d)

Co-authored-by: Stan Ulbrych <89152624+StanFromIreland@users.noreply.github.com>

Author: miss-islington

Doc/library/codecs.rst

@@ -1395,7 +1395,11 @@ encodings.
 |                    |         | It is used in the Python  |
 |                    |         | pickle protocol.          |
 +--------------------+---------+---------------------------+
-| undefined          |         | Raise an exception for    |
+| undefined          |         | This Codec should only    |
+|                    |         | be used for testing       |
+|                    |         | purposes.                 |
+|                    |         |                           |
+|                    |         | Raise an exception for    |
 |                    |         | all conversions, even     |
 |                    |         | empty strings. The error  |
 |                    |         | handler is ignored.       |