Explain how to port existing code

Author: vstinner

Doc/whatsnew/3.13.rst

@@ -120,6 +120,31 @@ Removed
 
 * :pep:`594`: Remove the :mod:`!cgi`` and :mod:`!cgitb` modules,
   deprecated in Python 3.11.
+
+  * ``cgi.FieldStorage`` can typically be replaced with
+    :func:`urllib.parse.parse_qsl` for ``GET`` and ``HEAD`` requests, and the
+    :mod:`email.message` module or `multipart
+    <https://pypi.org/project/multipart/>`__ PyPI project for ``POST`` and
+    ``PUT``.
+
+  * ``cgi.parse()`` can be replaced by calling :func:`urllib.parse.parse_qs`
+    directly on the desired query string, except for ``multipart/form-data``
+    input, which can be handled as described for ``cgi.parse_multipart()``.
+
+  * ``cgi.parse_multipart()`` can be replaced with the functionality in the
+    :mod:`email` package (e.g. :class:`email.message.EmailMessage` and
+    :class:`email.message.Message`) which implements the same MIME RFCs, or
+    with the `multipart <https://pypi.org/project/multipart/>`__ PyPI project.
+
+  * ``cgi.parse_header()`` can be replaced with the functionality in the
+    :mod:`email` package, which implements the same MIME RFCs. For example,
+    with :class:`email.message.EmailMessage`::
+
+        from email.message import EmailMessage
+        msg = EmailMessage()
+        msg['content-type'] = 'application/json; charset="utf8"'
+        main, params = msg.get_content_type(), msg['content-type'].params
+
   (Contributed by Victor Stinner in :gh:`104773`.)