[3.14] gh-144133: Add a warning to the encodings.punycode documentation (GH-151812) (#151923)

(cherry picked from commit 7ec70e1)

Co-authored-by: Stan Ulbrych <stan@python.org>

Author: miss-islington

Doc/library/codecs.rst

@@ -1396,6 +1396,14 @@ encodings.
 | punycode           |         | Implement :rfc:`3492`.    |
 |                    |         | Stateful codecs are not   |
 |                    |         | supported.                |
+|                    |         |                           |
+|                    |         | .. warning::              |
+|                    |         |                           |
+|                    |         |    The decoding and       |
+|                    |         |    encoding algorithms    |
+|                    |         |    scale poorly, so       |
+|                    |         |    limit the length of    |
+|                    |         |    untrusted input.       |
 +--------------------+---------+---------------------------+
 | raw_unicode_escape |         | Latin-1 encoding with     |
 |                    |         | :samp:`\\u{XXXX}` and     |